He needs to be eating weeds, flowers and opuntia cactus. Pumpkin vine leaves and flowers are great, Hibiscus leaves and flowers, dandelion and clover. Okra, chocho vine leaves, purslane, watercress.
Dehydration is a real killer condition. Start right away by making him stay in a tub of warmish water up to where his top and bottom shells meet. At least one hour every day or twice per day.
Hi. Sorry for this situation. May we see pictures of your turtles and their setup so we might have some ideas about how best to help? You are already doing well by keeping them clean.
It's desperately in need of immediate intervention with an intensive program of proper care all round. Hydration, temperatures, lighting, substrate and diet! It's dying.