Okay i have 2 Oscars. They are under 6in so i had them in a 55 by themselves. ( Yes i have 120 not set up yet for when or if they get larger). Anyway i bought a 3in channel cat for my pond but figured id grow it out with the os in the 55 for the winter and then it would be in the pond 500 gall btw. Well bad idea as this Blue Channel Cat was an excuse my lang but an *******. It fin nipped the bejesus out of my o's.. 1 more than the other. Abe the red 1 is a bit aggressive and didnt take much but Michonne the black one got her tale fine nipped badly and went into a stressed out depression state. So i returned the cat. Abe mostly healed up but Michonne did not and then it started getting worse. Thinking fin rot set in because im dumb and skipped a wc well was 2 days late So i moved them to a 29 gallon i had to treat them. Cheaper. Anyway the tank was cycled and fine very clean. Left them for a week in this tank and i noticed Michonnes tail fin was getting worse but wasnt getting better. So i tried api E. M. eryth.... Well it killed my cycle. Her tale looks worse.. This is my wifes fish. She loves it and so do i and the kids. Very cool personable fish that i will take any advice to get her better. Please help. Ps i do have a cycled filter on the 55 i could put on. I did a wc last night brought it down from 4 to 1. Used ammo lock and quick start too. Ill put more pics up after work....they are scared of me now...all that moving around i guess....2 weeks ago i could hand feed em now nope.