Test your ammonia, nitrite, nitrate and pH. If those are all fine try adding a filter for additional oxygenation or dropping your water level to increase splashing or just adjusting the spray bar to increase surface agitation.
When you changed the water, did you match the temperature and pH of the tank water? Remember the dechlorinator? Just tossing out ideas.
Maybe the food caused an ammonia or nitrite spike and you need to change more water. The tests will tell you if that is the case.
IME that's normally a case of low oxygen levels in the water. Clean the filter media and impeller/pump in tank water. Dirty impellers/pumps reduce flow and oxygenation. You may also need to add another filter if there is not much there already.