Under sink RO systems all use the same core technology, so the differences that matter are in how they store water, how many stages they carry, and whether anybody independent has verified the claims. Those three questions separate genuinely different products from the marketing that surrounds them, and they are worth answering in that order.
Tanked systems: the traditional design
A tanked system runs the membrane slowly and banks the output in a pressurized bladder tank, so the faucet delivers at a usable rate whenever you open it. It needs no power, which means it keeps working during an outage, and it is mechanically simple with decades of parts availability. The costs are space, since the tank is bulky, and the fact that water sits stored between uses, which is why tanks should be sanitized when filters are changed. A tank also loses its air charge over time and needs occasional checking.
Tankless systems: compact and powered
Tankless units use a booster pump and a higher capacity membrane to produce water on demand, dispensing directly rather than from storage. They free up most of the cabinet, deliver water that has not been standing, and generally waste less to drain because the pump maintains good pressure across the membrane. In exchange they need a power outlet, stop working in an outage, contain electronics that can fail, and fill a large pot more slowly than a full tank does. For small cabinets and small households they are usually the better trade.
Stage counts and what they really mean
Systems are advertised by stage count, but stages are not equal and more is not automatically better. Every system needs sediment prefiltration, carbon prefiltration to protect the membrane from chlorine, the membrane itself, and a post carbon polish. Additional stages are usually a second carbon, a remineralization cartridge or a UV lamp. A well built four stage system outperforms a poorly built seven stage one, and each extra stage is another cartridge to buy. Judge by what each stage does and how much media it holds, not by the number on the box.
Certification is the only claim you can check
Manufacturers describe removal in impressive terms, and the only way to verify any of it is independent testing to the relevant standards: NSF/ANSI 58 covers reverse osmosis systems, with specific claims such as lead or arsenic tested individually. Certification is granted for a named model with named cartridges, so replacement filters from another supplier take you outside it. Look up the model in the certifier's own database rather than trusting a logo on the page, and treat a general claim of laboratory tested with no standard named as no claim at all.
Running costs and parts availability
The purchase is the small part of ownership. What matters over years is how many cartridges the system takes, how often they need changing, whether they are a common standard size or proprietary, and whether the maker will still supply them. Proprietary quick change cartridges are convenient and lock you to one vendor; standard housings take cartridges from anyone but lose certification if you stray. Decide which you prefer before buying. Our under sink RO comparison guide sets out the practical differences between the formats.
Questions people ask about under sink ro systems
Is a tankless RO system better than a tanked one?
It is more compact and delivers fresh rather than stored water, but it needs power and fills large containers more slowly, so the right choice depends on your cabinet and household.
Do more stages mean better filtration?
Not necessarily. Media quality and bed volume matter more than stage count, and each extra stage is another cartridge to buy and replace.
What certification should an under sink RO system carry?
NSF/ANSI 58 covers reverse osmosis systems, with individual contaminant claims certified separately, and the model should be verifiable in the certifier's own database.
Can I use third party replacement cartridges?
Physically yes with standard housings, but a system's certification applies to the named model with its named cartridges, so substitution voids that verification.