Thanks all for your help.
I believe Smudge has passed away. Earlier today he was floating sideways at a weird angle, (see photo), and about 10 mins ago I went to check up on him and he was upside down at the bottom. I flipped him over and there is no movement, his legs are rigid and his mouth is slightly open. I'll leave him in the tub and check on him tomorrow morning to be sure, but I'm pretty positive he's gone. I can't help but think about what I did wrong to have caused this to happen to him. He was my first axolotl, and he had a lot of personality. I'm really sad that he ended up spending his final days in a tiny tub rather than in his roomy 40gal with his tank mates.
I currently have his tank mates in a larger tub as well, I drained their tank and I plan on starting over, possibly adding a 3d background that I made. They will be in there until the cycle is done, doing 50% water changes daily, 100% every 2-3 days, I think. (it's about 5x the size of tub that Smudge was in.)
I just hope that I succeed with them where I failed with Smudge.