Even though you may have a filter overkill to combat ammonia buildup in your aquarium, that doesn't rule out water changes. You still must do some water change on a weekly basis at the minimum. You could do a 50% change or could go for a 75% if possible. Try to avoid pushing this for more than a week.
Feed your fish less to avoid excess ammonia. If you find they are not eating properly, that means the water is bad. Probably the filter is clogged, or bio load is too much for it to handle. If you got the cash, go in for some ammonia test kit and test the water every 2-3 days.
So to make things simple, if you suffer from less water supply:
- Keep as less fish as possible.
- Keep small fish that don't pollute the water.
- Lots of filters - Use, power filters and sponge filters if your on a budget. If not, go for a canister that's designed for a tank bigger than yours. This is known as an over kill, but in a scarce water condition, this will be good thing.
- Feed less. It depends on what fish you keep. Don't starve them. Get to know its minimum requirements and feed that amount to it daily.
- Choose proper non-messy foods like commercial pellets.
- If you have good lighting or your tank receives good daylight, throw in some hardy plants like Hydrilla, which will help a great deal in improving water quality.