So I recently purchased a new tank for my new beloved betta Dawn, very pretty white-blue betta, so I could move him from his old tank to a bigger one (double the size of his old 2.5 gallon). His old tank, of which I have drained already, was kinda dirty and the filter was too strong. However, during the move (of getting out the old filter/plants), I noticed the majority of the end of his tail is missing. I don't know how, but I suspect the filter or fin rot. I immediately moved him to a smaller tank, roughly a 3/4 gall that I usually use for separating baby guppies during water changes, and drained the old tank. I can't move him to the new tank for another 24 hours (because of tank sealant drying). 1. What should I do? 2. The filter was strong, but not all of his tail is missing. Any idea of what it may be?