First some aquarium details (can't fill it in on my profile since this is my first post): I have a 20 gallon semi-planted with 2 platies and 4 cory's. I recently moved from a 30 gallon tank because I had some space restraints (about two weeks ago). So this last weekend I had 8 platies, and since then I've had one or two die each day. The first symptom is swimming around with clamped fins, then they'd start being unable to swim very well (like laying upside-down or floating tail up), and usually by the next day they'd be dead. I checked all my water parameters with the AP master set, and all is good. I've had the original 3 platies for about 6 months, and the rest are their offspring. I've never had any die before this, so it's quite alarming. The corys seem totally fine. I thought maybe it was the stress of moving tanks, but they seemed totally fine when I got them in the new tank, up until they started dying. I got new plants for my tank, so maybe some disease was transmitted via those? The last two still seem healthy. No clamped fins and still very active, eating, etc. Any help is appreciated!