Cuttlebone is a calcium supplement.
I've had good results using a reptile calcium power found in any pet store. I wet their pellets, dip them in powder, let them dry, then feed them to the turt.
What temp is your water?
Also, when I've rehabbed little RES with shell rot, I actually fed them in a small tupperware with some calcium powder in the water with them, hoping they'd drink some too.
I'm finicky and it may sound a little excessive, but I've rescued some hatchlings who were knocking on death's door and brought them back. They grew big, green, and fast.
I brought them outside for 30 minutes at a time, twice a day, out if the water. The trick is, you want the shell to be exposed to straight sunlight to dry out as if they were really basking in the wild.
Hopefully you live in a warm climate, as I wouldn't recommend this otherwise.

