What kind of fish do you have? What size is their tank? What is the temperature, ph, ammonia, nitrate and nitrites in your tank? When was your last water change and how much did you change? How often are you changing your water?
I know it is a lot of questions, but in order to help you with your problem it would be really helpful if you could anseer them.
Usually gasping is a sign of illness or stress. Bad water conditions, like high ammonia, nitrite or nitrate can cause your fish to gasp. A good start is to do a 25% water change.
Lower oxygen levels in your water can cause gasping as well. Warmer water holds less oxygen. If you have a hang on back filter you can lower the water level to increase the surface agitation. This will add oxygen to your water. Adding an airstone can help too.
