An under-sink reverse osmosis system stores treated water in a small tank under the sink. The tank holds water on one side of a diaphragm or bladder and air pressure on the other. That air pressure is what pushes the stored water out to the faucet. When the air pressure drops, the system can still make water, but it delivers it poorly, and the change is easy to mistake for a clogged filter.
How the warnings show up
The first signs usually appear at the faucet rather than in the taste of the water. The flow may be strong when the tank is full and then fade quickly, so the last part of the tank comes out in a weak or sputtering stream. A glass may take much longer to fill than it did when the system was new. Some households notice that the tank seems to refill and empty in a shorter time than before, or that the faucet spits air after the tank has run low.
Notes that separate a flat tank from a clogged filter
A clogged filter usually reduces flow across the whole tank cycle, while a flat tank tends to drop flow mainly as the tank empties. Keep a simple note: how long a glass takes to fill when the tank is full, and how long it takes when the tank has been drawn down. If the gap between those two times grows over weeks, the tank is a likely suspect. Also note whether the faucet sputters at the start of a draw or at the end. Bring those notes to the provider, because they narrow the diagnosis before anyone opens the system.
Check the tank only with the water off
Before any hands-on check, shut off the feed to the system and drain the storage tank completely, following the manufacturer's steps. The air valve on the top of the tank can then be read with a simple gauge. Compare the reading with the air charge stated on the tank or in the manual. If you do not know the target, or if the tank cannot be drained safely, leave the adjustment to the installer rather than guessing.
What to ask the provider
Ask whether the tank was pre-charged at installation, what air pressure it should hold, and whether the shutoff and check valves in the line are working. Ask how the tank is checked during a routine service visit and whether that check is written on the service record. If a tank fails repeatedly, ask whether the diaphragm or bladder is the likely cause and what the replacement involves. A provider that can answer these questions in plain terms is easier to hold to the work than one that offers only a general service call.