Well, I never thought I'd have to ask this. I know all about the importance of banishing chlor(ine/amine) from my fish tank. I know all about the nitrogen cycle and fish chemistry... But my 4 year old sister clearly does not. My molly gave birth to 50+ fry recently. They seem happy and healthy! Until my little brother discovered one of them stuck in a small glass cup in his room. We eventually discovered the culprit of this unthinkable atrocity (After I put the 3/4-of-the-way-dead-looking fish in the hospital tank-actually-just-a-breeder-box-in-main-aquarium*, of course.) My sister said she just wanted a fish of her own (Heaven knows why she put it in our bro's room). We don't know how long the fish was in there. You bet we had a long talk about this, as this is not the first time she's endangered the lives of all my fish. * I assumed it'd be okay to keep it in the main tank because... it was just chlorine? There's no chlorine in my tanks as I always treat new water with (maybe slightly superfluous amounts of) conditioner. The poor baby fish seemed to have bounced back astonishingly well. Swimming around happily, eating itty bitty bits of ground up New Life Spectrum pellets... But now its tail fin seems to be... gone... Like it's just not there anymore. There's the base of the tail fin, but... no fin. It can still swim fine, and the fish is active, but where's the fin? To state the obvious: Not attached to the fish. How might I go about treating this? The fish's other fins appear to be deteriorating as well, and I'm determined to save it. One more question. I noticed the tiniest of tiny little white squiggles between the plastic of the breeder box and the glass of the main tank. At first, I thought it was dust or something. But then I noticed it moving. And I got super concerned. Do I have to deal with parasites now too? How do I fix it? Why can't life be simple, why can't I just have happy fish wahhh :'( Any insight at all is very highly appreciated. Thank you! I'll try and get pics up asap. Update: Picture of affected fish down below.