I recently got a new Wakin goldfish, (feeder tank rescue) and he seems really healthy except that he has a tendency to, as I refer to it, "flounder;" by which I mean he leans slightly to one side when he swims. There is no side in particular that is the problem, but when he swims he always leans to one side; usually it is the direction away from the back wall of the tank. He doesn't seem to have any trouble with his swim bladder (he can move up and down without using his fins), and he has no fin tears that might unbalance him. Could this be a parasite problem? I really don't know much about parasites, but seeing as he is from a feeder tank, it seems possible.
-The ammonia, nitrite, and nitrate levels are 0.
-The pH is 7.5
The problem has gone away over the past week (I did a 30% water change on Monday and Friday), but I would like to diagnose it if possible, so that if it happens again I know what is happening.
If anyone has any ideas at all, I would really appreciate it if you shared them; I am drawing a blank on this.