Hello, I have a blackmoor goldfish that is a year old, he is about 2 inches big. I recently had a problem in my tank with some plants that started decaying as soon as they were put in the tank and left a lot of decaying matter under the gravel. This made tiny white worms appear in the tank. I used a gravel cleaner to get all of the dead plant out of the tank and did at half water change. When I did the half water change I put 0.5ml of safe start into the tank to replace some of the bacteria I took out with the dead leaves and water as well as tap safe. Thats when I noticed that my fish had a mucus starting to grow on his gills, at first he was eating fine and swimming around as normal. I started to treat him with King British Disease Clear but after a day he had stopped eating all together and wasn't opening him mouth to breathe. Now after day three his gills aren't moving in and out and have a full coating of this cottony white growth and the inside of the gills are very red and he keeps coming to the surface for air. I'm taking water samples to the vet in an hour to get them tested so I don't know the exact figures but as far as a diagnosis goes the pet store have been very wrong in the past so I was wondering if anyone on here would know better.