Hey, in the last few days or so I've had several problems. Firstly, on Thursday night my bronze cory died from severe fin rot. I have been medicating for at least 2-3 weeks with melafix to try to solve this problem; but to no avail. Since Thursday, I have noticed that my other 3 bronze corys are developing fin rot too. Overnight, two of them have got drastically worse to the point where one of them almost has no tail anymore. I went to medicate them again today and on opening the hood of the tank, I found a black-skirt tetra dead against the filter.
I'm completely baffled as to why fish are suddenly dying. The tetra was completely fine from appearance literally 2 hours beforehand and was eating and swimming.
I had my water tested recently and it was spot on, the filtration has not been a problem and although the stocking isn't great (product of not reading up properly when I first got the tank) there are no major problems with aggression beyond the odd bit of chasing. The only thing I've done differently is add the melafix, could this be doing more harm than good?
Any ideas as to why my fish are getting fin rot/dying?

I had my water tested recently and it was spot on, the filtration has not been a problem and although the stocking isn't great (product of not reading up properly when I first got the tank) there are no major problems with aggression beyond the odd bit of chasing. The only thing I've done differently is add the melafix, could this be doing more harm than good?
Any ideas as to why my fish are getting fin rot/dying?
