Why are you keeping the FRT in such a low pH? These turts hail from alkaline conditions. I kept my 4 FRTs in a pH of 8.0 and higher. Their substrate was fine coral sand with the addition of aragonite rice granules. Are you offering your FRT calcium supplements and vitamins containing D3 (so your turt can absorb the calcium). And, my turts did better with the addition of some marine mix in their water. I kept them just shy of brackish conditions to help in preventing fungal infections to their leathery carapace. At first, I kept them in neutral fresh conditions and they kept having fungal infections to their nostrils. That stopped after the addition of salt. Also, your temp needs to come down to about 84F max.
Note, keep an eye on your FRT once he reaches about 2 years of age. One of my young turts bit a wedge-shaped chunk out of a 2ft AUL and the lungfish didn't survive the wound.
I see what looks like a mounted heater in pic #2. If it is, get a shield on it. You turt will try to get behind it at some point and it's carapace can be easily burned.
