Is floating normal for a Black Moor Goldfish?

i have three lovely goldfish, two veiltails and one black moor. i had the two veiltail fish first, but the larger one bullied the smaller, so i got a third, the black moor, to change the group dynamic in hopes of alleviating the bullying. now there is peace in my fish tank, and all three fish swim and eat and seem perfectly fine and healthy.
except for one thing about the black moor. he has the strange habit of floating on his side at the top of the tank. he doesn’t gulp for air, so the oxygen level is fine (besides, i have a filter, and the other two don’t gulp air either), and he doesn’t have any problems swimming normally when he moves, so i don’t think it’s a swim bladder issue. neither of the other two fish do this, and with the exception of his floating, they all seem perfectly healthy. His floating has no apparent correlation to their feeding, so i don’t think it’s gas. His swimming movements are energetic and normal,his appetite is also healthy. i can’t stress how perfectly healthy he seems, until i glance over at my tank and he’s suddenly floating at the top so still i think he’s dead. until he rolls back over and swims off around the plants in a perfectly normal manner. he couldn’t do a better job of playing dead than if he were trying to trick me.
the only possible explanation i’ve come up with is that he likes the warmth of the light. but i thought goldfish were cold water fish? the water temperature is fairly consistent, never below 65 or above 70 degrees Fahrenheit. He even has a light gold sheen that black moors get when exposed to warmer than normal water conditions.
all in all, his behavior doesn’t seem like his health is really suffering, i’ve just NEVER seen a fish behave this way before….
Kylie- i don’t have any way of measuring the ammonia, nitrite, and nitrate levels at the moment, but i don’t think they’re the problem. if a chemical imbalance was the problem, the other two fish would be affected, especially considering the fact that Black Moors are more tolerant of environmental changes than veiltails.
in truth, the tank is a bit small, just a 10 gallon for the three of them, but i change the water regularly, once every 3 days, regardless of how clear it appears.
also, i feed them simple goldfish flakes (top fin brand for now), not the sinking food, but i also hold the food under the surface of the water to allow it to get soggy so they don’t accidentally gulp air while eating.

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