Please Help!

My Black Moor George is acting strange. I’ve had him a three months and has recently had fin rot, but was getting better and his fins were healing nicely. But now seems worse than ever, he is now not eating and just floats in one spot in the tank (some time on his side), and is sluggish when he does swim. He gets excited as he always has when I put the food in (I use pellets), he eats it but then spits it back out, he’ll do this a few times and then just stops and floats off.
I have tried giving him treats (loves lettuce and cucumber which he normally loves) and get the same result.
He is in a 30 litre biorb which until recently he was sharing with my fantail (now in his own tank). The water is within parameters and is tested weekly, change between 30-50% every Sunday, and use stress coat to condition the water.
He has been like this for about a week now, I have been hoping that he would eat something but he’s not, he is now losing weight and I’m really worried about him.

If anyone could help that would be great.

Well he's in a too small tank its 40 liters per gallon.
I would try different processed foods. Or some peas shelled or some broccoli.

Just a little update I had to move George to the hospital tank with only a few inches of water, as him stopped swimming altogether and was floating and banging into things. He’s been in there two day and I have been feeding him flake food. He seems to be eating little bits but is still spitting any flakes of size out. I crumbled the flakes so he would eat something and now he is swimming (not on his side) and has a little more energy, I’m going to slowly increase the amount of water with the daily changes.

Any other advice or thoughts on why he can’t/won’t eat anything bigger than a crumb at a time?

Thanks for help and advice about food and tank size.