I read through the diseases listed in the sticky but none of them really jumped out at me.
I'll explain the problem first, specs for my tank are below.
One of my female red wag platies has been sick for weeks now. The day I was leaving for a vacation I noticed that she had a HUGE chunk gone from her tail fin, it was nearly gone, and she was staying at the bottom of the tank. My boyfriend was taking care of the fish for me and was terrified he was going to kill the healthy ones. I didn't have time to move her to a QT tank and explain to him what he'd need to do, so I had no choice but to leave her there and hope she pulled through.
When I got home from vacation (11 days later) she was still alive! The tail was growing back a little, but she was still mostly on the bottom, coming up only to eat.
As time has passed since then, about 3 weeks, she's not gotten any better. Her tail is not healing and growing back as fast as I'd expect it to. She's gotten very pale. Now she mostly rests in the plant branches at the surface of the tank and only moves when she's scared by another fish.
I also notices today that she swims on an angle. It has kind of seemed like her tail was somewhat paralyzed and that might have been causing the problems. Now she swims at an angle almost on her side.
I don't know what to do. I clearly waited too long to recognize the real problem, I thought she'd just gotten into a fish fight but now I think there's been a disease taking its toll on her.
10 gallon tank
Cycled, filtered, aerated, heated to 76
Real plants (java ferns, amazon swords, tiger lily)
Water changed - 3 gallons weekly, 50% change about once a month
Water treated with Prime
Fed API bottom feeder pellets and a tropical flake medley 1x per day, skipping 1 day per week.
3 peppered cories
2 female red wag platies
1 male sunburst platy
3 baby cross platies (they likely won't stay in this tank as soon as they're old enough to determine gender and I can figure out where to put them)
1 female betta (she is docile)
Thanks for any help anyone has to offer! I've had a couple neon tetras (in separate tanks) die of neon tetra disease - minus the slowness of the progression this time, it actually kind of seems like that.