What is a good aquarium setup for a freshwater stingray?

You need a filter for one. Your Nitrate needs to be below 20 The Nitrite needs to be 0 as does the ammonia which you didn't list. You also need a MUCH bigger tank... like... 180 to START with. You're having a hard time keeping your levels down because you don't have a filter and he's in a tiny tank. (Tiny for him anyhow)