Hi there, water change is just a temporary relief to reduce the Nitrate and Ammonia contents. But is a good way to reduce most of the harmful substances in the water.

After changing the water, check PH, TH, CH, NH4 (ammonia), NO2, NO3.

A good water condition boast health and immunity to the fish.

Add JBL Denitrol after each water change. When you get water from tap, don't forget to add anti chlorine.

Check your fish food brand, perhaps you should switch to something that give fish more nutrients and healthy.