sorry for your loss. im sure he had one fin in the grave to begin with and you arent to blame atall.. he looks like he was stunted and just neglected by the previous owner..
I did not fully read the original post.....Nothing you could have done and good on you for giving him a week so comfortable that he stopped fighting his ailments relaxed and eventually succumbed....Sorry for your loss, and keep saving neglected fish! They deserve some good life
Go test the tank you got him from....And then compare it to yours. The white looks like missing scales and things. When I have a fish die in the night and wake up to it, it always looks like your fish. There are a few things I suspect, like big fish stress (how long was the car ride?) Salinity, and just not adjusting to the new params. How did you acclimate him? By that I mean what all did you do before you put him in the tank.
The ride home was 20 minutes. The store's water has a PH of 7.2, mine is 7. I don't know the other params of the water. I acclimated him in the standard way (slowly putting my tank water in the bag, dumping out old bag water, etc.) Then I just left him alone, checking on him two or three times a day.
EpiC: I did the 50% water changes a day because of the meds and also in the past I've found that frequent big water changes can help a fish recover faster from things like torn up fins.
Thanks guys for helping me feel better, it really sucks to lose a fish.