Why is my tiger barb hiding?

All my tiger barbs about 8 of them are playing and one tiger barb is hiding in a ornament.He will only come out for food or he is bothered swimming in there ornament fine but it looks like hes breathing heavily. I have a filter and an air pump. Its a gallon is also 25 gallons. i also have a heater and the temp is set to about 75,76,77,78