Hi Danny, make sure the mata doesn't have any skin or shell infections. They usually can be treated topically with Betadine, Silvadene Cream, Gentian Violet, or Chlorhexidine.
Keep the pH in the range of 5 to 6 to prevent such infections and have good biological filtration to keep ammonia/nitrate levels low. Partial water changes 1 to 2 times a week is a must.
Since most matas will only accept live fish, parasite infestations need to be addressed a couple of times a year. I worm mine by hiding Panacur/Benzelmin paste and Flagyl on the end of turtlesticks and shoving the stick into live, spineless goldfish (use scissors to remove spines). The food stick is a bottle cap and I use the goldfish as a drug vehicle. If the mata is not eating, you will have to stomach tube it. Consult a vet if worming turtles is new to you.
I have been a matamata keeper for 10 years and you learn from the ones that live/die. Bacterial infections and parasites is what usually kills them.
Good luck with these awesome animals.
Minh Phi
California