Hello, I recently rescued this angelfish from a family member who had been keeping it in a half full, unfiltered, unheated 10 gallon tank for several months after the tank it was originally in broke. I have since moved it to a 40 gallon where it is currently the only resident. Given the conditions it was living in, I would be shocked if it isn't sick in some way, but I don't really know how to go about diagnosing it. The main things I notice about it is that it has very pale and translucent fins, it constantly hides in one corner of the tank, and it looks like it may have some fin rot. There was also a dead fish in the tank I rescued it from so I imagine the water conditions were awful. If anyone knows what issues could be affecting it (or if by some miracle it seems fine) I would appreciate the help.
Additionally, I am using an Aqueon Quietflow 75 to filter the tank, and am a little concerned it may be too large and have too strong of a flow. Should I swap it out for a smaller filter?
The pictures are of the tank that I found it in, and its current condition.