Breathing heavily, gasping, rapid gill movement, these are signs that something may not be quite right in your aquarium. A fish should not look as though it is fighting for oxygen. Obviously it will have to open its mouth but it shouldn't be a constant opening and closing motion. If you notice that your fish has started to struggle with its breathing then there are a few things to do at first. Firstly is your water in good condition? You check and it looks like it is. Check for ammonia, nitrite, nitrate and pH. If your water checks out okay make sure that your tank is being oxygenated probably. Check whether water is coming back in from your filtration, is it creating surface agitation? If not, it could just be a case of lack of oxygen in the water. Make sure your power head is agitating the water surface so oxygen mixes with the water.
Breathing heavily is often a sign of stress and if a fish is ill this is exactly what it will do. Fish can suffer from all different types of illnesses, from internal parasitic and bacterial infections, to organ disorders. Heavy breathing in fish isn't always serious, but it's something you shouldn't ignore.