My Oscar Fish Is Breathing Very Heavily
The warmer the water is, the less oxygen it will hold. This is why in the summertime when it's very hot you often see fish in ponds hanging just under the surface, often gulping for air. Check the temperature of your water, make sure that your heaters are not malfunctioning and heating the water too much. You're more likely to find the water increasing greatly in temperature during the warmer months of the year so it might be worth investing in an air pump so you are unable to increase oxygen levels in your water.
Unfortunately it may be a little more serious than just a lack of oxygen, or even toxins in 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.
Problems that occur in the fishes gills can be caused by such things as disease and parasites such as gill fluke. Fish with these conditions can certainly cause them to gasp at the surface. Always have a closer look at the fishes gill in this sort of situation. If there is a problem in this area, there may be a chance that they will be discoloured and sometimes slimy .
I would suggest you get a very good fish health book and read it properly. For instance, gill disease will make a fish appear to breathe heavily or open and close its gills more often. Fish can be awkward critters, very difficult to diagnose sometimes.