I was hoping to never post in this forum... but here I am. I woke up this morning to something on one of my fish and I need help identifying it. I'm pretty sure I jinxed myself because two days ago I was telling my wife that my proudest thing with the fish tank was the absence of disease. Background on the fish/tank: - Tank has been running since February, fishless cycled, fish added February 25. - Fish in question (harelquin rasboras) were in quarantine for 4 weeks with no concerns before being added on June 1. - No new additions to livestock (except what the shrimp produce) since then. Only additions have been plants and now running DIY CO2. - 40-60% water change weekly every Wednesday. - Dosing the tank with Stability every water change just in case, Flourish 2-3 times a week and nothing else. - 50% water change last Wednesday (4 days ago). Parameters this morning (after I noticed the problem): pH: 7.8 ammonia/nitrite/nitrate: 0/0/5 TDS: unchanged at 476 (same range as tap) temp: 75.2 (ranges from 75-79) (API master test kit) No other fish are showing any signs of anything for concern. I have a ready to go hospital tank I can set up if needed. I have inverts in tank that may be intolerant to some medicines to treat the whole tank (amano shrimp, red cherry shrimp, nerite snails and one african dwarf frog). Pictures of fish in question. It is only one fish, and only on one side at base of top fin and on tail. The spot at the front of the fin bulges out some.View attachment wQGdCg8.jpgView attachment iDQ5ni8.jpgView attachment t5ITEgJ.jpgView attachment pNbYh7R.jpg I am off to do a 40% water change while I wait for replies. Thank you all in advance.