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An under-sink reverse osmosis (RO) system is the most common way to get heavily filtered drinking and cooking water from one kitchen faucet. It pushes water through a semi-permeable membrane that rejects a large share of dissolved solids, and it pairs that membrane with sediment and carbon stages. It is a point-of-use system: it treats water for one faucet, not the whole house.
This guide compares six under-sink RO systems sold on Amazon, explains the differences that matter (tank vs. tankless, waste ratio, certification, remineralization), and covers where RO fits next to softeners and whole-house filters. Specs and certification claims come from each manufacturer’s Amazon listing; we have not run these systems ourselves, and we recommend confirming any certification in the certifier’s public database before buying.
Quick Picks
- Budget tank system: APEC Water ROES-50 (about $180), 5 stages, 50 gallons per day.
- Higher-output APEC: APEC Water RO-90 (about $220), 5 stages, 90 gallons per day, listed as WQA Gold Seal certified.
- Remineralizing, whole-system NSF/ANSI 58 claim: iSpring RCC7AK (about $200), 6 stages, 75 gallons per day.
- Tank system with leak detector: VIVA21 5-Stage (about $200), listed as WQA Gold Seal certified to NSF/ANSI 58 and 372.
- Tankless with smart faucet: Waterdrop G3P600 (about $540), 600 gallons per day, 2:1 pure-to-drain ratio.
- Permeate pump and UV stage: Home Master TMHP HydroPerfection (about $500), 9 stages, 1:1 waste ratio per the listing.
Where Under-Sink RO Fits (and Where It Does Not)
RO, softeners and whole-house filters solve different problems, and mixing them up is the most expensive mistake in water treatment:
- A water softener removes hardness minerals (calcium and magnesium) from all the water entering the house by ion exchange, which protects plumbing, fixtures and water heaters from scale. It is not a drinking-water purifier.
- A whole-house filter (usually sediment plus carbon) treats all the water for things like chlorine taste, odor and sediment. It generally does not reduce dissolved minerals.
- An under-sink RO system reduces dissolved solids and a long list of specific contaminants, but only at one faucet, and it produces water slowly through the membrane.
Many homes with hard water use a softener for the whole house and RO at the kitchen sink. Softening ahead of RO also reduces scale on the membrane. Our water treatment decision matrix compares all four approaches side by side, and whole-house reverse osmosis: is it worth it? covers when treating every tap with RO makes sense.
Start with a water test. If you are on a private well, test for bacteria and other contaminants before choosing equipment; a standard RO system is not designed as a stand-alone disinfection system for microbiologically unsafe water, and your local health department can advise on well water.
What to Compare
Tank vs. tankless
Traditional RO systems store filtered water in a pressurized tank, because the membrane produces water slowly. Tankless systems use a pump and a high-output membrane to make water on demand. Tankless units take less cabinet space and avoid water sitting in a tank, but they need an electrical outlet under the sink and cost more. If your sink cabinet has no outlet, adding one is a job for a licensed electrician.
Gallons per day
The GPD rating is the membrane’s production capacity under the manufacturer’s test conditions. Real output depends on your water pressure, temperature and dissolved-solids level. Low incoming pressure reduces production and can increase waste; a permeate pump (like the one in the Home Master) or a booster pump helps in low-pressure homes.
Waste ratio
RO sends some water to the drain to flush rejected minerals off the membrane. Newer designs advertise lower ratios: Waterdrop lists 2:1 pure-to-drain for the G3P600 and Home Master lists a 1:1 waste ratio for the TMHP. Ratios depend on conditions, so treat listings as best-case figures.
Certification
NSF/ANSI 58 is the performance standard for reverse osmosis systems, NSF/ANSI 42 covers aesthetic effects such as chlorine, and NSF/ANSI 372 covers lead content in materials. Certification can apply to a whole system or only certain claims, and can come from NSF, WQA, IAPMO or other accredited bodies. Read the exact wording in each listing, then look up the model number in the certifier’s online listings.
Remineralization
RO strips out most minerals along with contaminants. Some systems add a post-filter stage that puts some minerals back for taste. It is a taste and preference feature.
The Six Systems Compared
| Model | Stages | Listed GPD | Tank | Listed certification | Price (Oct 5, 2026) |
|---|---|---|---|---|---|
| APEC ROES-50 | 5 | 50 | Yes | Not stated in listing | about $185 |
| APEC RO-90 | 5 | 90 | Yes | WQA Gold Seal | about $220 |
| iSpring RCC7AK | 6 (incl. alkaline) | 75 | Yes | NSF/ANSI 58 (whole system) | about $200 |
| VIVA21 5-Stage | 5 | Not stated in listing | Yes | WQA Gold Seal, NSF/ANSI 58 and 372 | about $200 |
| Waterdrop G3P600 | 8 | 600 | No (tankless) | NSF/ANSI 58 (TDS), 42, 372 | about $540 |
| Home Master TMHP | 9 (incl. UV) | Not stated in listing | Yes | UV stage EPA-registered | about $500 |
APEC Water ROES-50
The APEC ROES-50 is a conventional 5-stage tank system rated at 50 gallons per day. APEC lists reduction of arsenic, chlorine, lead, fluoride and heavy metals, and says it includes the filter set, a brushed nickel faucet, tank and installation kit, with quick-connect fittings and color-coded tubing. APEC recommends filter changes every 6 to 24 months depending on incoming water. Price on October 5, 2026: about $180.
APEC Water RO-90
The APEC RO-90 uses the same 5-stage layout with a 90 gallons-per-day membrane. The listing says it is WQA Gold Seal certified, uses a lead-free faucet and John Guest quick-connect fittings, and has a maximum feed water pressure of 85 psi. If your home pressure runs higher than that, a pressure-reducing valve is needed; a plumber can check. Price on October 5, 2026: about $220.
iSpring RCC7AK
The iSpring RCC7AK is a 75 gallons-per-day tank system with a sixth alkaline remineralization stage. iSpring states the entire system is certified to NSF/ANSI 58 and lists reduction figures including TDS (93% to 98%) and lead (over 98.9%). The faucet installs from above the countertop, and the first-stage housing is transparent so you can see sediment buildup. Price on October 5, 2026: about $200.
VIVA21 5-Stage
The VIVA21 5-Stage system (model V21RS5S01) is listed as WQA Gold Seal certified to NSF/ANSI 58 for RO performance and NSF/ANSI/CAN 372 for lead content. It ships with faucet, tank, tubing, installation kit and a leak detector, which is a useful extra for a system that lives in a closed cabinet. VIVA21 lists a 1-year limited warranty. Price on October 5, 2026: about $200.
Waterdrop G3P600
The Waterdrop G3P600 is a tankless 8-stage system rated at 600 gallons per day with a 2:1 pure-to-drain ratio. Waterdrop lists NSF/ANSI 58 certification for TDS reduction, NSF/ANSI 372 for lead-free materials and NSF/ANSI 42 for chlorine, taste and odor. The faucet shows TDS and filter life. It requires an outlet under the sink, and the unit measures 18.12 x 5.67 x 17.72 inches, so measure your cabinet first. Price on October 5, 2026: about $540.
Home Master TMHP HydroPerfection
The Home Master TMHP is a 9-stage tank system with a non-electric permeate pump, two-pass remineralization, an iron prefilter and an EPA-registered UV stage, per the listing. The permeate pump is the reason Home Master claims a 1:1 waste ratio and faster tank fill. The UV stage is a polishing step. Price on October 5, 2026: about $500.
Installation and Upkeep
Most under-sink RO systems are sold as homeowner-installable, with push-fit tubing. The job usually involves a cold-water supply connection, a drain saddle on the sink drain, mounting the faucet (sometimes drilling a countertop or sink), and placing the tank or unit. Some jurisdictions require an air-gap faucet or specific drain connections, so check local plumbing code. If you are not comfortable with any of it, or have stone counters to drill, hire a licensed plumber.
Plan for ongoing costs. Replacement intervals vary by system and water quality: APEC suggests 6 to 24 months for its filter set, and iSpring sells replacement kits labeled from 6-month to 3-year. Check replacement filter prices before buying, since the cheapest system is not always the cheapest to own.
Frequently Asked Questions
Does an under-sink RO system soften water?
It reduces hardness minerals in the water it produces at that one faucet, but it does not protect the rest of the plumbing, appliances or water heater. For whole-house hardness, a softener is the tool; see water softener vs. whole-house filter.
Tank or tankless?
Tankless systems save cabinet space and make water on demand but need an outlet and cost more. Tank systems cost less and work without electricity, though they take up more room.
How much water does RO waste?
It depends on the system and your water pressure. Listings range from the Waterdrop’s stated 2:1 pure-to-drain ratio to Home Master’s 1:1 claim, and older designs send more to the drain. Low water pressure increases waste.
Do I need to remineralize RO water?
It is a taste preference. Some people like the flavor of remineralized water; a remineralization stage like the iSpring’s or Home Master’s adds minerals back.
How do I check a certification claim?
Search the model number in the certifier’s online listing (NSF, WQA or IAPMO) and check which standards and contaminants are covered.