Diagnosis
Not normal for settled-in use. That’s a classic sign that the alkaline remineralization filter (the 6th stage, usually the last canister) is brand new and needs a serious flush, or it’s failing/contaminated.
How to Fix It
If it’s a new filter, it needs a massive flush. The media inside is calcite and other minerals that have fines. Turn on the faucet and let it run for a solid 30-45 minutes to flush all that out. Seriously, go do something else. The water may run white or cloudy at first—that’s the fines. After the flush, the taste should clear up to a clean, slightly smooth taste.
If That Didn't Work
If the taste persists or appears on an old filter, the filter media might be exhausted or, worse, contaminated. Check the filter’s age. The AK filter should be replaced every 6-12 months or 2000 gallons. If it’s past that, replace it. Also, ensure the filter is installed in the correct direction (flow arrow pointing towards the faucet).
Heads Up
The water from the alkaline stage will measure a higher TDS than the pure RO water from stage 5. That’s the minerals being added back. Don’t panic if your TDS meter reads 20-50 instead of 5-10; that’s the point.