Hi, I wouldn't worry too much. Shell curling is generally
due to overfeeding. I have 5 turtles ranging from 10 years
to 7 years to 1 year old. In my opinion you are overfeeding
based on your post. My painted turtles slow down with eating
a couple of times a year but mainly in the fall. They never
stop completely but are more picky and eat less. You didn't
mention how large your turtle was?

Just let him be picky and don't worry about the calcium
supplementation. I worried in the beginning but soon found
out that this behavior doesn't last. I would be very careful
about how many fish you are feeding him. Be strong and don't
give in. Try some different things too. Dry catfood soaked
until soft is a favorite of mine. Also, make sure you are
using a UVB light source like the Reptisun 5.0.

I have attached a picture of one my sliders. This girl
is 7 years old. It's hard to believe she was ever a tiny
green hatchling.

Hope this helps
Linda