I have a 20 gallon aquarium with 1 adult platy, 1 juvenile platy and 1 guppy. Recently I've noticed that my adult platy has this weird lump near her top fin, I'm not sure what it is or how to treat it. Since noticing this I've also noticed when she eats bubbles come out of her mouth and she hides in the moss most of the day only really comes out to eat. Before she got sick I had a molly who's tail was split and I was dosing the tank with API Melafix (for about 3-4 days). She passed away and I did a recommended water change and that's when I noticed this spot appear on my platy. I did a 30% water change yesterday and she came out and swam for a bit before hiding again. Does anyone know what this might be?