Under-Sink Reverse Osmosis Brands and Models: Tanked, Tankless, and Remineralizing

Brands making under-sink reverse osmosis systems for US kitchens include Viomi, APEC, iSpring, Express Water, Home Master, Aquasana, Pentair, AquaTru, Cloud, Culligan, Kinetico, Waterdrop, and Frizzlife. Their models split into tanked systems, which store purified water in a pressurized tank, and tankless systems, which use an electric pump to make water on demand, as the Viomi V8 PRO and Viomi V6 PRO do. Within both groups, some models add a built-in remineralization stage that puts minerals back after the membrane. Two of these brands, Culligan and Kinetico, sell and install through local dealers.

Brands at a Glance

Tanked Under-Sink RO Brands

A tanked system fills a pressurized storage tank slowly, then delivers stored water quickly when you open the faucet. Most tanked models need no electricity, and several use industry-standard cartridge sizes.

A few brand notes:

• APEC, iSpring, and Express Water build their five-stage systems around the same classic sequence of sediment and carbon pre-filters ahead of the membrane, with a carbon polish afterward. APEC lists industry-standard 10" filter compatibility for its RO systems, excluding tankless and some models; iSpring's RCC series uses 10" pre-filters.
• Aquasana and Pentair use their own cartridge formats. Aquasana's SmartFlow cartridges twist out without disconnecting hoses; Pentair's are click-in.
• Home Master and Cloud add remineralization to a tanked design. Home Master's permeate pump model lists a 1:1 waste-to-pure ratio, and Cloud's battery-powered system works without an outlet while tracking TDS at three points through its app.
• Culligan and Kinetico come with a local service relationship. Both work through local dealers who test your water, then install and maintain the system.

Tankless Under-Sink RO Brands

A tankless system uses an electric booster pump to push water through a high-output membrane as you draw it, so there is no storage tank under the sink. These systems need an outlet, and most pair with a smart faucet that shows water quality and filter status.

Output and pure-to-drain figures are each brand's published ratings. GPD means gallons per day and expresses rated daily purified-water production.

Viomi V8 PRO

The Viomi V8 PRO is Viomi's 800 GPD tankless system for households that draw a lot of purified water through the day. Viomi splits its 9 stages between just two cartridges (PCB composite and RO) and rates it at a 3:1 pure-to-drain ratio. Real-time TDS and filter life appear on the LCD smart faucet, and usage history and replacement reminders live in the Viomi app. In the box are one RO filter and two PCB filters, so a spare PCB is on hand from day one. A separate line in standard tubing can carry its water to a refrigerator or ice maker. Its dimensions are 16.3"L × 5"W × 12.8"H.

Viomi V6 PRO

The Viomi V6 PRO shares the Viomi V8 PRO's compact body, 9-stage filtration, 3:1 ratio, LCD smart faucet, app support, and refrigerator connection, with a 600 GPD membrane. It brings the same smart monitoring to kitchens that are well served by a 600 GPD rating. Its RO replacement cartridge is made specifically for the Viomi V6 PRO and twists and pulls out without tools.

The other two Viomi models in the table fill different needs. For mineral-added water, the Viomi M1 combines remineralization with a 1000 GPD rating and a touchscreen faucet that reads out tap and purified TDS. The Viomi B6W is a 600 GPD, 7-stage system that works with a standard faucet and the Viomi app.

Brands With Built-In Remineralization

RO membranes remove most dissolved minerals along with contaminants. The World Health Organization's background document on TDS notes that water with extremely low TDS may taste flat. Remineralizing models are an option for buyers who prefer water with minerals added back. Two approaches exist:

• Built in as a stage: Viomi M1, Waterdrop X12 and G5P500A, Frizzlife PD600-N and PD800-N, iSpring RCC7AK and RO500AK-BN, APEC ROES-PH75 and ROTL-600PH, Express Water ROALK5D, Aquasana SmartFlow RO, Home Master TMAFC-ERP, and Cloud RO.
• Optional cartridge: Culligan's Mineral Boost options and Kinetico's Mineral Plus.

Adding minerals back raises the TDS reading compared with the RO water before that stage. The Viomi M1's touchscreen faucet shows both tap and purified TDS, and its purified reading sits above that of standard RO water because minerals are deliberately put back. For a deeper comparison of mineral stages, see this overview of RO brands with remineralization filters.

Power, Faucet Hole, Drain and Space: What Every Under-Sink RO Needs

Installation planning covers power, faucet space, drainage, cabinet clearance and inlet conditions:

1. Power. Tankless systems need an outlet. Viomi models run from a GFCI-protected circuit. Where a garbage disposer is installed, give the purifier a separate always-on outlet instead of the switched one that powers the disposer. If the switched outlet is the only one under your sink, plan for a dedicated always-on outlet before installation day.
2. A faucet hole. Most systems use a dedicated drinking faucet. Viomi calls for a 1" to 1.5" countertop hole, Waterdrop recommends 1" for the G3P600, Cloud needs 0.5" to 1.25", and APEC's standard RO faucet uses a 1/2" hole (1" for an air-gap faucet). If your sink has a soap-dispenser hole, some brands suggest using that.
3. A drain connection. Every RO system sends concentrate to the drain, usually through a small hole drilled in the drain pipe (1/4" on Viomi models).
4. Cabinet space. A tankless unit is one box; a tanked system adds a tank. Measure the cabinet and account for the disposer and existing pipes.
5. Inlet pressure and water source. Published inlet ranges differ by brand: the Viomi V6 PRO and Viomi V8 PRO accept 14.5 to 72.5 psi, the iSpring RCC7 45 to 70 psi, and the pumped RCC7P 30 to 70 psi. Viomi RO systems are designed for municipal tap water.

For a closer look at how tanked and tankless designs trade space, flow, and efficiency, see tankless vs. tank RO systems.

FAQ

Should I choose a tanked or tankless under-sink RO?

Choose tankless if cabinet space is tight, you want water made fresh on demand, and you have an outlet under the sink. Choose tanked if you want stored water ready at the faucet; models such as the iSpring RCC7 also run without electricity. Most systems of either type use a dedicated faucet hole, though Culligan's ClearLink PRO option switches your existing faucet between RO and tap water, and every RO system needs a drain connection.

Do under-sink RO systems need an electrical outlet?

Tankless systems do, because an electric pump drives the membrane. Many tanked systems, including the iSpring RCC7, Express Water RO5DX, and Kinetico K5, rely on household water pressure alone. The iSpring RCC7P adds a booster pump to a tanked design and needs power for it, and Culligan's Smart RO runs on a 12 VDC supply.

Can one under-sink RO feed both the kitchen faucet and a fridge line?

Many can, through a separate line. The Viomi V8 PRO, Viomi V6 PRO, Viomi M1, and Viomi B6W all support a refrigerator or ice-maker connection using a length of standard tubing added at install, and the Viomi M1 uses its "Multi-device Mode" in the app for that setup.

How do I tell an under-sink RO from an under-sink carbon filter?

An under-sink RO system has an RO membrane among its stages and sends concentrate to a drain. Under-sink carbon filters, such as the Viomi Under-Sink Carbon Filter, Aquasana's Claryum Direct Connect, and Waterdrop's 10UA series, filter water without an RO membrane or drain line. Aquasana says of its Claryum unit, "this is not purified water, it is filtered water."

How the Under-Sink RO Field Sorts Out

Under-sink RO brands sort into a few clear groups once the kitchen's priorities are set. Compact cabinets that want water made on demand, with TDS and filter life shown at the faucet, line up with tankless smart-faucet models such as the Viomi V8 PRO (800 GPD), Viomi V6 PRO (600 GPD), Waterdrop G3P600 and G3P800, and Frizzlife PD600-N. Buyers who prefer minerals added back after the membrane can compare the Viomi M1, Waterdrop X12, Frizzlife PD800-N, iSpring RCC7AK, and APEC ROES-PH75. With no outlet under the sink, tanked systems that run on household water pressure, such as the iSpring RCC7, Express Water RO5DX, and Kinetico K5, or the battery-powered Cloud RO, fit the space. For standard-size replacements, the APEC ROES-50 takes industry-standard 10" filters and the iSpring RCC7 uses 10" pre-filters. Culligan and Kinetico pair their systems with a local dealer who tests the water and handles service. Whichever group fits, the power, faucet hole, drain, cabinet, and inlet pressure checks above decide how well a specific model suits your kitchen.