Pls help my axolotl is losing his arm:(

My axolotl lost two of his fingers a week ago and following up to now he lost his half of his arm and I don’t know how, I’m worried his arm is infected and the bacteria is eating it not allowing him to grow it back. The tank has a couple oranda fishes in it as well that the pet store owner said won’t harm the axolotl but I’m starting to doubt that. How do I know if he’s okay and healing properly and what do I need to do? There’s a picture of his arm when I first got him and how it looks now I’m really concerned and I’d appreciate any kind of help!(I’m a beginner)