My goldfish when he stops swimming is going vertical, tail up. Paramaters were decent, .1 ammonia 0 nitirite 5-10 Nitrate. I did a water change after testing. His other tankmates are fine, however they were added only about a week ago and the goldfish has been doing this for a couple days now. I suspect the ammonia is from the new additions. The tank is a new acquisition, used tank about 2 weeks ago, with Nitrate showing it appear to be cycled. It had obviously been sitting outside for a little while before I got it, had some spider webs in it that I cleaned out, and a bunch of dirt lining everything, cleaned out as well.
He can still swim fine, although is often at a slight angle with one side leaning, and is able to get to the surface for food just fine.