There is 2 reason fishes breath fast.
Firstly it could be lack of Dissolved Oxygen (DO) in the water, this can be cause be warming of the tank water by whatever reason eg. power head over heating, lights, etc or chemicals. If it's the first reason, you got to remove the source of heat production or add fans or chiller and if it's the second you have to dilute the chemical by some water change.
Secondly it could be due to parasitic infection that effects the gills, thereby causing the fish to breath harder and faster. Identify the problem and medicate as required.
I believe the problem is likely due to warm water.