Little copper floater, why?

I have 3 9cm copper axies. They were in a 60cm tank (until they get bigger) with sand and a filter. A PVC pipe, hide and some plants. Also a feeding jar, but they don't see how it works...

Anyway. One of them started to float while in the tank. I wasn't sure if 'sand' was good for axies that small. (At that moment they were about 6,5 cm...)
So I placed them in a bare tub. Did water changes daily and he didn't float.
After a few weeks they're in the tank again and one of them is floating again.

I don't know why. The current of the filter isn't too strong, temperature is 19C and I cycled the tank for about a month before I got them. Water parameters are 'axie okay'. Is it the sand? I'm thinking about putting them in a smaller tank (40 cm one), without sand but with the filter and the hide, plants and PVC pipe. But if it's something else, let me know.

Oh, about the floating lotl. Looks like he has an air bubble in his stomach or something. I can't see it, but when it swims down he goes up again.

Skip the sand until they're bigger.

Floating and air bubbles in juveniles is usually from not getting enough food.

Thanks. No sand for them
They have big full bellies, they're eating a lot actually
(That's good)