Hi all, I am sort of new to this red eared slider world as I got two little guys (or gals, who knows

) only about four months ago. They were given to me as a gift and I know they were very little, both slightly bigger than a quarter (illegal as I learned later). Fast forward 4 months later, one of them is just awesome, super alert, super hungry, super huge. The other one is the trouble. He hasn't really grown too much, he's now about three times smaller than his bro, doesn't eat for days on no end and isn't very active, mostly sleeps. The biggest problem - aside from his troubling eating habits and size compared to his bro - is that he clearly can't even get to the bottom of the tank if he tries. His butt is obviously floating and everytime he does want to eat, he can't even get to the bottom to grab the food. I of course 'serve it to him' on something little higher. He seems to also be trying to 'cough out' air mainly right after eating, kind of dry heaving under water with his mouth wide open. I'm really worried because some days he looks like he's really struggling and is giving up on his little life he has. This has been happening for about 4 - 6 weeks. Some stats for you guys to evaluate my situation: tank is about 10 gallons, water about 3 - 4 inches, two lamps UVA and UVB, I change water couple of times a week and try to pick up leftover food to keep tank cleaner. I feed them mini floating food sticks or blood worms once an day, have given them some raw fish as well, they have rocks to hang out on and under, I turn lamps on in the morning and turn them off for the night. Any advice would be amazing.
Thank you for reading and I appreciate all feedback!

Martina