Why do my turtles keep getting sick?

It is very irritating. I have 2 turtles both under a year old. They are in a 55 gallon tank. I use a Fluval 305 canister filter and the water is very clean. Also I have a heater that I have set to 75F degrees for the water temperature, and my thermometer is on par with it (towards the end of the day the temperature goes up from the heat lamp being on all day, max temp being 82F). They have a heat lamp as well as a basking platform. They eat fine, they don't have shell rot either. Just every other night as i go to bed i see one of/or both of them making croaking noises and blowing bubbles. I have been giving them sulfa dips and their syptoms go away then come back. What am i doing wrong?