10 gallon tank
Heated to 76F
live plants (a variety, all aquatic)
tank has been set up for 9 months
3 adult platy (2 female 1 male)
3 juvenile platy (1 "teenager")
3 spotted cory catfish
1 female betta (non aggressive)
All fish have been in tank and living together for at least 5 months, no problems.
I do a 25% water change weekly
gravel vaccuum 2x month
ammonia - 0
nitrite - 0
nitrate - ~30 (gets up to 30 before water change, down to 20 after)
One of my female red wag platies has an odd greyish patch just at the front base of her dorsal fin. It kind of looks like all the scales have fallen off, but it's hard to tell. Her side fins (the small ones she uses to steer) seem like one may be a bit ragged, but I can't say I know for sure they haven't always looked that way.
Appetite seems fine, behavior seems fine, just the odd patch. It's definitely not white and it's not fuzzy. There may be another small patch near her gill on one side, but again I might be seeing "symptoms" that aren't really there.
I read through the lists of diseases posted by Lupin on here, none of them sounds right. Blue Slime Disease sounds closest, but I haven't noticed flashing.
Help? I've isolated her, but I don't know how to treat and I don't know if I need to treat the whole tank. The scaleless cories make it tricky.