You may also help keep toxins such as ammonia and nitrites low by feeding a smaller amount of food perhaps once each day. Any levels of ammonia or nitrites will be harmful so Water changes daily might be needed rather than once a week. It will depend on what the daily test results show.
Would also be careful to protect the bacteria (good kind) in the filter material, by ALWAYS cleaning it in old aquarium water you take out at water changes or ,in a bucket of dechlorinated water. Tapwater should not be used to clean filter cartridges,sponges,pads etc. The chlorine or chloramines found in most water supplies will kill off the (good Bacteria) that consumes the toxins mentioned (ammonia,nitrites.)
The most important medication in your fish medicine cabinet is.. Clean water.