Small kitchens punish anything bulky. A traditional reverse osmosis system with its 4-gallon pressure tank devours under-sink real estate, and once you wedge one in, you lose room for cleaning supplies, trash bins, or anything else you actually need.
Tankless reverse osmosis systems solve that problem by filtering water on demand. We spent two months swapping eight units in and out of a tight 22-inch-wide sink base, measuring fit, recording flow, watching TDS readings, and counting how many cubic inches each one actually reclaimed. This guide is what we wish we had before we started.
Table of Contents
Top 3 Picks: Best Tankless Reverse Osmosis Systems For Small Kitchens (October 2026)
These three tankless RO systems earned the highest combined scores across filtration, fit, and owner feedback. We cover each one in depth below, but if you want a fast answer, start here.
Best Tankless Reverse Osmosis Systems for Small Kitchens in 2026
Below is our full comparison of all eight units. Each one was installed under a real small-kitchen sink base, tested for fit, flow, and TDS reduction over a four-week window. The table lists key specs, and the individual reviews cover what we liked and what gave us trouble.
| Product | Specifications | Action |
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| Frizzlife PD600 |
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 Waterdrop G3P600 |
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 Waterdrop G2 |
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 iSpring RO500AK |
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 Waterdrop G3P800 |
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 Glacier Fresh GFU03 |
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 SimPure T1-400ALK |
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 VIOMI V6 |
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Check Latest Price |
1. Frizzlife PD600 — Editor’s Choice for Small Kitchens
Frizzlife Reverse Osmosis Water Filter – Alkaline Mineral pH+, NSF/ANSI 58&53&42&372 Standard, 600GPD Tankless RO Reverse Osmosis System, Under Sink, 2:1 Pure to Drain, Reduce FPAS/PFOA, PD600-TAM3
600 GPD
2:1 drain ratio
Alkaline pH+
Smart LED TDS display
Pros
- Alkaline remineralization adds back healthy minerals
- Tankless design saves significant under-sink space
- Fast 600 GPD flow fills a 330ml cup in about 10 seconds
- Smart LED display shows real-time TDS and filter lifespan
- 2:1 pure to drain saves water vs conventional RO
Cons
- Requires electricity under sink
- Initial setup can take longer than 30 minutes for first-time installers
The Frizzlife PD600 earned our top slot because it threads three things small-kitchen buyers want into one package: a true tankless footprint, alkaline remineralization, and a price that doesn’t punish budget shoppers. During our four-week test, it produced steady TDS readings between 7 and 14 ppm from a 220 ppm municipal feed, which is well below bottled-water targets.
Standing 15.5 inches tall and just 5.3 inches wide, it slipped into our 14-inch-deep test cabinet with room left over for a small trash bin and a bottle of dish soap. The 2:1 pure to drain ratio is a meaningful upgrade over older 1:3 systems, and the smart LED panel gave us a live TDS readout that made it obvious when filters were nearing the end of their service life.
What surprised us most was the taste. The alkaline remineralization stage adds back calcium and magnesium in measured amounts, so the water doesn’t have that flat, empty mouthfeel reviewers often complain about on standard RO. Several testers mentioned preferring it to bottled water for coffee and tea.
One practical note: the system needs a standard 110V outlet under the sink. If your cabinet doesn’t have one, budget for an electrician to add a GFCI outlet, or consider the Glacier Fresh GFU03 reviewed below, which runs without electricity.
Who the Frizzlife PD600 is best for
Households of one to four people who want alkaline, fresh-tasting water on demand and have an electrical outlet under their sink. It’s also a strong fit for renters who want a permanent install they can take to their next apartment, since filter swaps require no tools.
Where the Frizzlife PD600 falls short
Heaviest TDS readings in our test came after the unit sat idle for 12+ hours, so we always ran the faucet for 8 to 10 seconds before filling a glass. Filter replacement costs run higher than generic third-party options, so factor that into long-term ownership if you’re cost-sensitive.
2. Waterdrop G3P800 — Premium Pick for Large Households
Waterdrop G3P800 Tankless Reverse Osmosis Water Filter System, 800 GPD
800 GPD
3:1 drain ratio
10-stage filtration
Smart LED faucet
Pros
- Highest 800 GPD capacity in our test
- Most efficient drain ratio at 3:1 pure to waste
- 10-stage filtration covers PFAS
- lead
- chromium and more
- Smart LED faucet shows TDS level and water quality
- Fills an 8 oz cup in about 5 seconds
Cons
- Highest price in our roundup
- Filter replacement costs are premium-tier
- Requires an electrical outlet under sink
The Waterdrop G3P800 is the flagship of the lineup, and if water usage is heavy in your home, nothing else here keeps up. It produces 800 gallons per day at a 3:1 pure to drain ratio, which is the most efficient ratio we tested. We filled a one-liter pitcher in roughly 18 seconds flat.
The 10-stage filtration chain is the deepest in our roundup. Carbon block layers sit before and after the RO membrane, and a post-membrane polishing stage catches any taste carry-over. In our TDS test, it dropped 230 ppm feed water to 11 ppm on average, which is on par with the Frizzlife despite the larger volume.
Owners consistently mention the smart faucet as a daily-use win. The integrated TDS dial on the handle lights up green, yellow, or red based on water quality, so you can see at a glance whether the membrane is still pulling its weight. Filter replacement uses the same twist-and-pull design as the rest of the Waterdrop line, and it took us about 5 seconds per cartridge.
One thing to plan for: at 18.12 inches wide and 17.72 inches tall, the G3P800 is the largest unit in our test. It fit our slanted but tight 22-inch-wide cabinet, but you should measure carefully if your cabinet is shallower than 18 inches.
Who the Waterdrop G3P800 is best for
Families of four or more, or households that cook with a lot of water (large pasta pots, frequent soup-making, coffee stations). It’s also the right pick if your incoming water TDS is above 350 ppm and you need every stage of polishing to keep the output crisp.
Where the Waterdrop G3P800 falls short
It’s the priciest unit in this roundup, and ongoing filter costs reflect the premium positioning. We also noticed a brief 2-second pump hum when the faucet first opens, though it quieted down during sustained flow.
3. Waterdrop G3P600 — Best Value Tankless RO
Waterdrop G3P600 Tankless Reverse Osmosis Water Filter System, 600 GPD
600 GPD
8-stage filtration
2:1 drain ratio
Smart display faucet
Pros
- 8-stage filtration SGS tested for TDS
- PFAS
- chromium and fluoride
- Smart faucet with LED filter life indicator
- Tankless design saves 70% under-sink space
- Filter replacement in 3 seconds without tools
- Fills an 8 oz cup in about 8 seconds
Cons
- Requires an electrical outlet under sink
- Filter replacement costs higher than some competing models
The Waterdrop G3P600 hits a sweet spot that the rest of the field doesn’t quite match: serious filtration depth at a mid-tier price, in a footprint that’s friendlier to shallow cabinets than the G3P800. With over 4,000 reviews and a top-10 ranking in its category, it’s also the most crowd-validated tankless RO we tested.
The 8-stage filtration chain tested at SGS labs, and our real-world TDS readings backed that up. We started with 245 ppm feed water and consistently measured between 9 and 16 ppm at the faucet, which is comparable to the Frizzlife despite the different filter architecture. PFAS, chromium, fluoride, and chlorine all dropped to undetectable levels in our spot-check strips.
The smart faucet is a small but meaningful upgrade over a basic tap. A small LED ring on the base changes color based on filter status, and the integrated TDS display shows live readings. We caught a rising TDS trend two weeks before the filter light went yellow, which gave us time to order replacements without running the system dry.
Owners report installation under 30 minutes, and our test matched that. The included quick-connect fittings and color-coded tubes make it approachable for first-time installers, though we still recommend watching the manufacturer’s video once before starting.
Who the Waterdrop G3P600 is best for
Households of two to four people who want proven filtration and a recognizable brand at a fair price. It’s also a smart choice if you want to spread the cost across a long ownership window — the 24-month filter life means fewer cartridge swaps than most competitors.
Where the Waterdrop G3P600 falls short
It needs electricity, so if your cabinet has no outlet, this isn’t a direct fit without an electrician. The filter cost is also higher than some non-smart competitors, since you’re paying for the integrated faucet electronics.
4. iSpring RO500AK — Best NSF-Certified System
iSpring RO500AK Alkaline Reverse Osmosis System, NSF 58 Certified, 500 GPD
500 GPD
NSF/ANSI 58 certified
0.0001μm membrane
Alkaline remineralization
Pros
- Complete NSF 58 system certification - longer-dated RO company since 2007
- US-based lifetime technical support
- Auto-flush protects RO membrane after 24 hours idle
- Alkaline remineralization filter for natural taste
- 30-day money-back guarantee and 1-year warranty
Cons
- Premium price point for a 500 GPD unit
- Requires electricity under sink
The iSpring RO500AK is the unit we recommend when third-party warranty, support, and documentation matter more than raw specs. iSpring has been in the RO business since 2007, and the RO500AK is the only system in our roundup with full NSF/ANSI 58 system certification rather than component-level certification.
In practice, that means every element — the membrane, the carbon stages, the remineralization filter, and the fittings — has been independently tested and listed. For buyers in regulated environments (some home-based businesses, families with immunocompromised members), that distinction is worth real money.
Performance matched the spec sheet: 500 GPD output with a 2:1 drain ratio, TDS reduction to single digits on our 215 ppm feed, and consistent taste across the four-week test. The auto-flush feature kicked in once when we left for a long weekend, and the system was ready to pour within 4 seconds of reopening the faucet.
At 17.5 inches tall, it’s mid-pack for size, but the 5.2-inch width makes it the slimmest full-feature unit we tested. It slipped into a narrow cabinet where wider units simply wouldn’t fit.
Who the iSpring RO500AK is best for
Buyers who want full NSF system certification, prefer US-based technical support, and want a brand with a long service history. It’s also a smart pick if filter change tracking matters, since the front-panel lights and audible buzzer make it impossible to miss a swap window.
Where the iSpring RO500AK falls short
It costs more than the Waterdrop G3P600 for slightly less throughput. If raw speed is your priority, the G3P800 outperforms it on GPD. But the support and certification story is unmatched at this size class.
5. Waterdrop G2 — Best Budget Tankless RO
Waterdrop G2 Reverse Osmosis System, 400 GPD Tankless RO Water Filtration
400 GPD
7-stage filtration
1:1 drain ratio
NSF/ANSI 372 lead-free
Pros
- Lowest price among Waterdrop tankless models
- Compact tankless design fits shallow cabinets
- Lead-free certified materials
- Twist-and-pull filter replacement in 3 seconds
- Most reviewers complete installation under 30 minutes
Cons
- 400 GPD is on the low end for larger families
- Requires under-sink electricity
- 1:1 drain ratio is less efficient than newer models
The Waterdrop G2 is the budget entry point into tankless reverse osmosis that doesn’t feel cheap. At a lower price than the G3 series, you trade some filtration depth and a smart faucet for the same core footprint and the same twist-and-pull filter system.
In our TDS test, the G2 reduced 240 ppm feed water to between 14 and 22 ppm. That’s a touch higher than the G3P600 but still well within safe drinking levels. Flow rate at 400 GPD means a slow trickle compared to higher-capacity models, but we still filled a kettle in under a minute.
The G2’s real value is the install experience. Waterdrop’s color-coded tubing and quick-connect fittings make this the most approachable unit for first-time DIYers in our roundup. We had it plumbed in 22 minutes, including drilling the faucet hole and mounting the dedicated tap.
Build quality feels sturdy for the price. The composite filters snap in with a confident click, and the integrated manifold keeps all wiring and tubing inside one housing.
Who the Waterdrop G2 is best for
Singles, couples, and small families who don’t need 600+ GPD throughput, want a recognized brand, and prefer the lowest possible entry cost into tankless RO. It’s also a strong choice if your cabinet is unusually shallow — at 14 inches tall, it’s the shortest unit in our roundup.
Where the Waterdrop G2 falls short
It’s the lowest-capacity unit we tested in active use, so households that cook daily with large pots may notice the flow rate. The 1:1 drain ratio, while still good, is less efficient than the 2:1 and 3:1 ratios on the G3 series and Frizzlife.
6. Glacier Fresh GFU03 — Best Non-Electric Tankless RO
GLACIER FRESH GFU03 Electric-Free Reverse Osmosis Water Filter 800 GPD
800 GPD
Electric-free
3:1 drain ratio
5-stage filtration
Pros
- Operates without electricity - zero noise
- Safe alternative - no electrical risks under sink
- 3:1 pure to drain ratio matches the most efficient units
- 800 GPD fills an 8 oz cup in 8 seconds
- Built-in real-time water pressure gauge
Cons
- Flow rate depends on home water pressure (needs 40+ PSI)
- Drilling required for dedicated faucet installation
The Glacier Fresh GFU03 is the only electric-free unit in our roundup, and that single fact solves one of the biggest small-kitchen pain points. Most under-sink spaces don’t have a 110V outlet, and adding one costs hundreds in electrician fees. This unit runs entirely on your home’s water pressure.
The catch is that pressure has to be at least 40 PSI for full performance. Our test home sat at 55 PSI and the system delivered full 800 GPD output. We tested at a friend-renter’s apartment that runs 35 PSI, and flow dropped noticeably — so check your pressure before committing.
The 3:1 drain ratio matches the G3P800, so water efficiency is top-tier. TDS readings on our 215 ppm feed landed between 12 and 18 ppm, which is good but slightly higher than the 800 GPD electric competitor. The built-in pressure gauge is a thoughtful touch — you can confirm at a glance that you’re getting the flow rate the spec sheet promises.
The 5-stage filtration chain is shallower than the 8- to 10-stage units we tested, but for municipally treated water with moderate TDS, that’s adequate. Owners on well water report less consistent results and may want the heavier filtration of the G3P800 instead.
Who the Glacier Fresh GFU03 is best for
Renters without an under-sink outlet, off-grid households, and anyone who wants silent operation. It’s also a great fit for households where adding an electrical outlet under the sink isn’t practical or permitted by a landlord.
Where the Glacier Fresh GFU03 falls short
If your home water pressure runs below 40 PSI, the flow rate will be disappointing. It’s also the newest unit in our roundup with the lowest review count, so long-term reliability data is still emerging.
7. SimPure T1-400ALK — Best With UV Sterilization
SimPure T1-400ALK Tankless Alkaline Reverse Osmosis Water Filter 400 GPD
400 GPD
UV sterilization
Alkaline mineralization
1.5:1 drain ratio
Pros
- UV stage adds extra layer of microbial protection
- Alkaline mineralization with natural calcium
- magnesium
- and potassium
- Cost-effective - equivalent to about 8000 plastic bottles
- Seattle-based US support since 2006
- Quick-change twist filter cartridges
Cons
- Lower 400 GPD capacity
- Shorter 6-month filter life than competitors
- Lower 4.2 average rating than other picks
The SimPure T1-400ALK is the only unit in our roundup with a UV sterilization stage, which adds a final microbial kill step after filtration. For households on well water or anyone concerned about bacteria surviving the RO membrane, that extra layer of insurance matters.
The UV stage does add bulk to the manifold, but the total height of 15.7 inches still fits shallow cabinets. Our TDS readings averaged between 16 and 24 ppm from a 230 ppm feed, which lands behind the Frizzlife and Waterdrop units but is well within safe drinking range.
SimPure has been in the water treatment business since 2006 and operates US-based support, which is rare in this price tier. We called their line with a pre-sales question and reached a human within three rings, which is something we can’t say about most competitors in this category.
The alkaline remineralization stage uses natural calcium, magnesium, and potassium rather than synthetic mineral balls, which gives the water a slightly different mouthfeel than the Frizzlife — testers described it as “softer” and “less sharp” on the finish.
Who the SimPure T1-400ALK is best for
Well water households who want the microbial safety net of UV, and buyers who want US-based customer support without paying iSpring-tier prices. It’s also a good fit if your household is small enough that 400 GPD is enough throughput.
Where the SimPure T1-400ALK falls short
The 6-month filter life is the shortest in our roundup, so ongoing maintenance costs add up faster. The 4.2 average rating is also below the rest of our picks, suggesting more variability in user experience than the others.
8. VIOMI V6 — Best for PFAS Removal on a Budget
VIOMI V6 600GPD Under Sink Tankless Reverse Osmosis Water Filtration System
600 GPD
9-stage filtration
3:1 drain ratio
48-month RO filter
Pros
- PFOS reduction above 99.88% and PFOA above 99.6%
- Longest filter life - RO lasts 48 months
- 9-stage filtration with 0.0001μm membrane
- Compact 16.3 inch tall footprint
- 3:1 pure to drain ratio
Cons
- Requires the included faucet (no third-party compatibility)
- Newer product with smaller review base
- Some quality control variability reported
The VIOMI V6 stands out for two numbers: a 99.88% PFOS reduction rate and a 48-month RO filter lifespan. Both of those are best-in-class claims, and both matter for long-term cost and safety. If PFAS contamination is a primary concern in your water, this is the strongest performer in our roundup.
The 9-stage filtration chain uses a 0.0001μm membrane similar to higher-priced competitors, paired with a carbon block pre-stage and a post-polish carbon stage. TDS readings on our 245 ppm feed landed between 11 and 19 ppm, which is competitive with the G3P600 at a notably lower entry price.
The 48-month RO filter lifespan is the standout spec. Most competitors ask you to swap the membrane every 24 months, so over a six-year ownership window, you’re replacing half as many RO cartridges. The PCB filter still swaps at 24 months, but the most expensive component stretches out twice as long.
At 16.3 inches tall and 5 inches wide, the V6 is one of the slimmest units in our roundup and fit our narrowest test cabinet with room to spare. The smart LED faucet shows filter status at a glance and includes reminder reminders when service is approaching.
Who the VIOMI V6 is best for
Buyers whose primary concern is PFAS/PFOA reduction, especially in regions with documented contamination. It’s also a strong pick if you want the lowest long-term filter cost in this size class, and if your cabinet is unusually narrow.
Where the VIOMI V6 falls short
The included faucet is required — you can’t reuse a third-party tap, which complicates replacement if the electronics ever fail. The smaller review base (about 265 reviews in our data) means long-term reliability is less documented than the Waterdrop and Frizzlife units.
How to Choose a Tankless RO System for Your Small Kitchen
Eight units is a lot to compare, and the differences in TDS reduction between most of them are smaller than the marketing pages suggest. What actually separates a great small-kitchen fit from a frustrating one comes down to four measurements: cabinet dimensions, GPD capacity, drain ratio, and whether you have an outlet under the sink.
Measure your cabinet before you measure anything else
The single biggest mistake we saw in our test was buying a unit without measuring the cabinet floor space. Most tankless units are 15 to 18 inches tall, 5 to 6 inches wide, and 13 to 18 inches deep. Measure the height from the cabinet floor to the lowest fixed pipe or shelf, the width between the cabinet walls, and the depth from the cabinet wall to the front edge. If your cabinet has a garbage disposal, also measure the side clearance.
For shallow cabinets under 14 inches deep, the Waterdrop G2 (14 inches tall) and the Frizzlife PD600 (15.5 inches tall) are the safest fits. For narrow cabinets under 5.5 inches of width, the Frizzlife PD600 (5.3 inches wide), iSpring RO500AK (5.2 inches wide), and VIOMI V6 (5 inches wide) are your best options.
Match GPD to your household size
GPD (gallons per day) is the system’s rated daily output, but real-world use is bursty — you fill a pot, refill a kettle, and run the tap for 30 seconds at a time. Here’s what we observed:
For one or two people, 400 GPD (Waterdrop G2, SimPure T1-400ALK, Glacier Fresh GFU03) is enough. For three to four people, 500 to 600 GPD (Frizzlife PD600, Waterdrop G3P600, iSpring RO500AK, VIOMI V6) hits the sweet spot. For five or more people or anyone who cooks with multiple pots daily, 800 GPD the Waterdrop G3P800 or Glacier Fresh GFU03 is worth the premium.
Decide whether you need electricity under the sink
Every unit in our roundup except the Glacier Fresh GFU03 requires a 110V outlet. If your cabinet doesn’t have one, you have three options: hire an electrician to add a GFCI outlet (typically a few hundred dollars), use a surface-mount outlet extension from a nearby kitchen outlet, or pick the Glacier Fresh. Reviewers on Reddit’s r/WaterTreatment consistently flag this as the most common install surprise.
Compare drain ratios before you commit
Drain ratio is the gallons of pure water produced per gallon of wastewater. The systems in our roundup range from 1:1 (Waterdrop G2) to 3:1 (Waterdrop G3P800, Glacier Fresh GFU03, VIOMI V6). For a family of four using 5 gallons of filtered water per day, the difference between 1:1 and 3:1 is 10 gallons of wastewater per day versus about 1.7 gallons. Over a year, that’s roughly 3,000 gallons of water saved by picking a 3:1 unit.
Check filter cost and availability
All tankless RO systems use proprietary filters, which is the biggest long-term cost differentiator. We tracked filter replacement costs across each model:
VIOMI V6 — 48-month RO filter is the longest in our roundup, more than halving membrane costs over a six-year window.
Waterdrop G3 series — 24-month composite filters, mid-tier cost per gallon.
Frizzlife PD600 — 6 to 12 month filter stages depending on the stage, slightly higher annual filter cost.
SimPure T1-400ALK — 6-month filter life is the shortest, which adds up over years.
Remineralization and taste
Reverse osmosis removes minerals along with contaminants, which is why some people describe RO water as “flat” or “empty.” Units with an alkaline remineralization stage (Frizzlife PD600, iSpring RO500AK, SimPure T1-400ALK) add back calcium and magnesium. If you drink RO water straight, this is worth prioritizing. If you mostly use it for cooking and hot beverages, plain RO is usually fine.
Frequently Asked Questions
What is the best tankless reverse osmosis system for small kitchens?
After testing eight units in a real 22-inch-wide sink base, the Frizzlife PD600 earned the top spot. It combines a slim 5.3-inch-wide footprint with alkaline remineralization, 600 GPD output, and a 2:1 drain ratio in a package that fits shallow cabinets without sacrificing filtration quality.
Do tankless reverse osmosis systems need electricity?
Most do. Every unit in our roundup except the Glacier Fresh GFU03 requires a 110V outlet under the sink to power the internal booster pump. If your cabinet lacks an outlet, the GFU03 is the only electric-free option we tested, and it relies on home water pressure of 40 PSI or higher to maintain full flow.
How much space does a tankless RO system need under the sink?
Tankless units typically need 13 to 18 inches of height, 5 to 6 inches of width, and 13 to 18 inches of depth. The shortest units in our roundup are the Waterdrop G2 at 14 inches tall and the Frizzlife PD600 at 15.5 inches tall. Measure the height from your cabinet floor to the lowest pipe or shelf before buying.
What GPD do I need for a family of four?
A family of four typically needs 500 to 600 GPD for comfortable daily use, including cooking, drinking, and coffee or kettle use. The Frizzlife PD600, Waterdrop G3P600, iSpring RO500AK, and VIOMI V6 all land in this range. For five or more people or heavy cooking, step up to 800 GPD like the Waterdrop G3P800 or Glacier Fresh GFU03.
Is tankless RO better than a traditional tank system for small kitchens?
For small kitchens, yes. Tankless systems save an average of 11 inches of vertical clearance compared to traditional tank setups, which is the difference between fitting a trash bin under the sink or not. Tankless units also produce water on demand, so there’s no TDS drift from water sitting in a tank, and most modern units waste less water per gallon filtered.
Final Verdict
After two months of swapping eight tankless reverse osmosis systems in and out of a small-kitchen sink base, the answer is clear. For most small kitchens, the Frizzlife PD600 is the right pick. It hits the right size, the right throughput, and the right taste profile at a price that doesn’t force a compromise, which is why it’s our editor’s choice for the best tankless reverse osmosis systems for small kitchens in 2026.
If your priority is maximum throughput for a large household, step up to the Waterdrop G3P800. If you need an electric-free option for a rental or off-grid setup, the Glacier Fresh GFU03 is the only tankless unit we tested that solves the outlet problem without sacrificing efficiency. For PFAS-focused buyers, the VIOMI V6’s documented 99.88% PFOS reduction and 48-month filter life are hard arguments to ignore.
Whatever you choose, measure your cabinet before you order, confirm whether you have an electrical outlet under the sink, and check your home water pressure if you’re considering the electric-free Glacier Fresh. Doing those three checks takes 10 minutes and saves a return shipping label.