Hi guys, I am new to the hobby, I am really excited to have gotten a couple of black moors goldfish on January 8th 2024. I used to own one when I was a kid, and now 25 years later, I decided to purchase a couple. Unfortunately, things haven't started good. I purchased a 55 gallon tank, educated myself on tank cycling, medications, etc, to the best I could, but ended up making mistakes. My set up includes a HOB filter, a sponge filter, and an extra airstone. I use Seachem prime, and I read that using Seachem Stability for the first 7 days, it is safe to add fish to the tank on the first day. I ran the tank for about a week before adding the 2 black moors, (got them from Petsmart and I think they are about 2 months old). I purchased an API test kit and tested everything before adding the fish on the first day. PH 7.8, ammonia 0, nitrate and nitrite 0. I made the mistake of having my HOB filter running at max flow and I had the LEDs on as well. Just 2 hours after placing the 2 black moors in the tank, they started to develop this white fuzzy material on their tail and fins. The next day they had a white slime covering their body, and their fins were frayed and seemed to be rotting away. One of the fish was just lethargic on the bottom barely moving and just gasping every 10 seconds. I went to a pet store and showed them a picture, the person said it was a bacterial infection and suggested a treatment of Kanaplex. After 2 rounds of treatment there was a significant improvement, the fish started swimming and I could see the white fuzz becoming less aparent. I completed the 3 final dose and they seemed alot better, and even started to eat. After, they had a white string fecal matter, and sometimes they would just 'flash' swim very fast for a second and slow down and I thought they had a parasite. I was suggested to put Metroplex on their food. They were okay and improving for almost 2 weeks, fins appeared to be regenerating. About a week ago I did a 80% water change, the day after the water change I noticed his top fin clamped. It has been 4 days and he spends most of the time at the top barely moving. I seen him with erratic movements against the glass and rubbing against when I turn on the LED. I read that flukes cause this. But I haven't seen him doing that again, he doesn't move around much. The physical sign that I see, is this white substance on the side, it started off in one side, and now its on both. I can't tell if its fuzzy, it doesn't appear to be. His fin is always clamped. I started another round of Kanaplex 2 days ago, and today I started the 2nd dose. I don't seen anything wrong with the water paremeters. The other fish appears normal so far... Sorry for the long text and for bad English (not first language). I created several accounts of websites and don't get much help. I have several medications at home, prazinquel, ich x, metroplex, kanaplex, aquarium salt, paraguard, polyguard. I really appreciate your assistance!!