I thought about posting this in disease, but I'm not sure it's a disease.
Below is a picture of Leo (the platy with the tattered fins) We awoke this morning to see her not acting well and with this red gash-looking thing on her side. To me it looks like she hit herself on a decoration, but her gills look rather red too, so I'm not sure.
I can test for
ammonia etc... if you want, but it's been 0 since we cycled our tank about 6 months ago. I've never had an abnormal reading. The red gills would normally make me think ammonia, but she has a lot of red in her coloration, and none of the other fish have red gills.
Anyway what do you think this mark is?