I've made many improvements to her habitat, its about 13 gallons...

It has a Fluval U1 filter system (made for 15 gallon tanks), an aerator (under the mountain on the left), a heater which maintains the water at 78F, and a basking area on the right. The basking area is set up with a heat lamp and a UVB light.

Submarine basks probably 8-10 hours a day (we have the lights on during daylight hours), but also loves to swim. She is quite active, and interacts with us quite a but. She is less and less timid as time goes on.
I've been cleaning her tank and replacing the filter ever 2 or 3 months which seems to be working out pretty well. The tank was just cleaned today, which is why the temp is a little warm with the fresh water I put in this morning.
We've been feeding her 5 pellets every other day, and red or green leaf lettuce about once a week. We've also been getting her a feeder fish to hunt once a month (she LOVES it!). She eats very well!
The problem is here carapace, its continued to deteriorate and I don't know why. Maybe you can help me identify things I could do differently to help it improve, or maybe I need to get her some medical help. Please LMK your thoughts.
Here is a picture of her from 6 months ago.

And these are from this morning.




She's keeping her eye on me



