Swollen, constipated, still eats/swims/is alert?

Hi, I have a seven year old melanoid axolotl who lives with an albino tank mate of the same age, both males and very friendly towards each other. Their tank is 75 gallons with many hides, larger rocks (not able to be consumed), and a fine sand substrate. I have hard water but treat it with API stress coat and do 50% water changes twice a month. I also have a cartridge filter with very established media.

Yesterday, I found my melanoid on the bottom where he usually is, and he even reacted to my movement and the light as if he were about to get fed. The problem was that his stomach had become swollen gradually for about three days, and yesterday it was too much to ignore. I don't think anything hard is inside, as there aren't any protrusions on either side of his ribs. The swelling is symmetrical/spherical, and bigger than his head. I put both axolotls in the fridge overnight until today when I could do a 100% water change and removed all substrate to prevent a possible ingestion again, if that is what this is. They haven't had an issue with the sand before, as they have lived on it their entire lives. The rocks are bigger than them, so he couldn't have eaten one.

What could this be? I'll keep him in the fridge until I see signs of improvement, while the albino will get to go back into their clean tank.

Hello oriissy and welcome to the forum.

Would you be able to post some pictures of your axolotl? That would be very helpful.

Hi! What is his diet like? Hope your problem gets resolved!!