How do I treat a goldfish that is lethargic, nomally sits at the bottom of the tank and doesn’t swim around?

I’ve had two goldfish for about two years, one common goldfish and a comet goldfish. I’ve never had any problems with the common goldfish (I’ve read they are very hardy fish), but the comet goldfish started floating a bit and having problems swimming around. About 3 months ago I noticed that he was having problems swimming straight and his back-end was floating more than his head so basically he had his tail up and his head down. Recently (about 1 1/2 months now) he has been very lethargic and either floats near the surfice of the tank or sits at the bottom of the tank with his head down an his tail slightly higher. He doesn’t swim around anymore and tends to just go to a corner and stay there unless he’s being fed. The only time he swims around and seems lively is when I feed them, but still has a bit of trouble swimming normally as his tails floats higher than his head, but still seems lively at least when being fed. They were in a 30 litre tank (approx 7 gallons) when the problem started, but 1 month ago I moved them to a 120 litre tank (31us gallons or 27uk gallons) as they were getting too big for their tank and I thought having more space would help with his problem, but he istill very lethargic and just sits at the bottom of the tank or floats near the surfice of the tank and does’t swim around (still with his tail a lot higher than his head nearly vertically) I tried not feeding him for a few days (3) and the i fed him peas (2) for a couple of days and there seem to be no improvement so I continued feeding him flake food, but his time I started soaking it in tank water first so it wouldn’t float and would sink to stop him from taking in air when he feeds hoping that this will stop his buoyancy problems. I’ve ben feeding him this way for about 1 1/2 months now and he’s still the same. I the last week or so I have noticed that his poop is long thin and white and I’ve heard that it is a sign of an interial bacterial problem. His poop also has air bubbles in it and floats to the top of the tank. Today I noticed he suddenly got a single big white spot on his side and it looks like it’s got fluff comming out from it. it looks like a big white fluffly dimple.

The tank is 120 litres as I said and only the two goldfish live in it. The ammonia levels were nil when they moved to the tank as it was well cycled and the nitrite levels were also good. the nitrate levels were a big high, but I made sure i made a %40 water change before tranfering them to the new tank so I don’t think there’s a water problem. It has been quite hot recently and the tempreture in the tank has been 72 – 80 degrees. I made sure the water from the filter drops into the tank creating a lot of bubbles and also introduced an airstone for extra oxigen. Yesterday I used a tonic solution to keep the fish healthy and to help with constipation and to relieve swim bladder symptoms, this tonic has salt in it to help relieve stress on the fish and help them heal.

I’m worried about his symptoms though because he’s now lethargic (sits in a corner or near the surfice nd doesn’t swim around), long thin white poop, buoyancy problems (his head down and tail up) and now suddenly he’s got that big fluffy spot on his side.

Does anybody know what it might be? Please help.
Please only answer if you have a fairly clear idea of what it might be and have had simmilar problems with your own goldfish. That way you won’t waste your or my time. Thanks

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