I watch my tanks every day multiple times a day and I don't know how I missed this. One of my panda Cory's and quarantine is missing almost its whole tail and has white fuzzy stuff on its tail and fins and is laying on its side. There's another one that's not doing so good and all the other ones are active but I could see a little bit on their fins. I'm still really new so I'm not good with fish diseases so I assume it's fin rot? And I did a large water change and put kanaplex in the tank. Could it be something else? Is there anything else I need to do or look for? How does this happen so fast!?