My guess is:
1) You are using test strips, which don't test the water very well (registering the water as fine when it's in the middle of spiking)
2) You are overfeeding them
3) They are suffering from internal parasites, which they got from the store you bought them from
4) You may be interacting with them too much, causing them to stress out
5) One or more of the fish is a bully
6) You might have very delicate fish that should be handled by someone more experienced
7) The water temperature is wrong
8) The system is overstocked
9) They might require more or less salt in their tank
The fact is, I don't know, because you haven't given us any basic information, like:
1) The size of the tank
2) The type of system (freshwater, brackish, saltwater)
3) The types of fish you have
4) The water parameters (temperature, pH, ammonia, nitrite, nitrate, yes I want specific numbers)
You sound like you've done your research, which is a great start, and I want to help you, but I need those questions answered first. Thank you!