E sub question

I have several E. subloglosa that I have had about a year now, and I have been dealing with SCUD-type issues periodically throughout this period. I had a group of three that never had this problem that I sold, but these 5 seem to keep getting ill. I can get them healed up just fine, and they will be looking good, shells almost back to normal, then I start seeing infected spots reappearing on them. What am I doing wrong?

Water temp: 78 degrees.
Air temp: Around 78-80
Basking temp: 95, with UVB exposure as well
Water depth: 7" (turtles are 3.5")
Water volume (not tank size, actual volume): 100 gallons
pH: 8.2
Ammonia: 0
Nitrite: 0
Nitrate: 80
Substrate: None
Tank decorations: None
Filtration: Wet/dry with 20 gallon sump
Diet: Reptomin, trout chow, smelt, earthworms, pinky mice, shrimp, romaine, grapes.
Treatment used in past: 10 day once daily treatment of scrubbing shell with 1% iodine, letting dry, then rubbing silver sulfadiazine cream onto shell.

I have kept freshwater turtles for years and have never had SCUD issues until these guys, would love to find out what is going on here.