So we have a pretty healthy 30 gal aquarium (admittedly overstocked - but we're working on that) with a variety of mollies, platies, tetras, 2 cories a clown pleco and one giant goldfish (he makes us water change every other day because of his bio load). One molly in particular, Big Momma, all of a sudden started swimming erratically and sank to the bottom. Now she's acting like her tail is broken. She tries to swim up for air but she can't move anything but the fin. I can tell she's fighting, and this isn't the first time we've seen it - it happened to her mate a couple months ago and he died. Temp levels are stable at 78, we have been battling fungus (due to the overpopulation I'm sure, but she is fungus free and ammonia levels have remained safevas we've been diligent in keeping the tank as healthy as possible while trying to thin out the herd so to speak. Anyway, why is she acting like she broke her back/tail? We put her in the breeding net inside the tank so she can easily get air.