Juvenile axolotl floating and can't swim down

Hello! I'm a first-time owner and a little unsure of how to proceed, so any help would be very much appreciated!

I got my little guy - Wooper - about two weeks ago, I'm not sure how old he is but he's about 5 cm (2 inches) long, and things seemed to be going fine until yesterday. When I got home from uni he was floating along the top of the tank. Since I've been feeding him just a few inches below the water surface for the last few days, I assumed he was just waiting for his meal (and he proceeded to eat normally).

However, the floating hasn't stopped, and he's been struggling to get back down to the tank floor once in awhile, between rapidly swimming back and forth along the surface and hardly moving at all. He's also been spending a lot more time in the decorative plant than he used to, I think because it helps anchor him down somewhat.

Today hasn't been any different, every so often he'll attempt to swim down but then just float back up. For awhile he even wedged himself under the filter to help keep himself down (and that freaked me out a bit!). He doesn't seem to have any signs of stress that I've read about (the curled tail or flared gills), and still ate as he normally does today. He isn't floating tail-end up, which I read might be constipation or impaction, it's more of a balanced horizontal sort of floating, or his head just a bit higher than the rest of his body. If anything, he's a little paler than normal.

Other details:

• It's a 20 gallon tank, about half full with a filter and sand substrate.
• I feed him thawed bloodworms once a day.
• 25% water changes 1-2 times per week.
• pH = 7.5
• Ammonia = 0.5 (I know this should be at zero, but heard that may take some time until the tank fully cycles?)
• Nitrite = 0
• Nitrate = about 20
• I'm unsure what the temperature is, I'll go out and get a thermometer asap, though I'm not sure if that's the problem. Like I said he was fine until yesterday, and the temperature in the room he's in hasn't changed in the last few weeks.

I know I might be jumping the gun a little here - and apologize for the small novel! - and that it might be nothing (I read some juveniles just like to float?), but I feel very worried nonetheless. I've read some threads about fridging, but I'm not sure if that's needed here yet, or if there's an age for that (most threads I've seen the axolotls are all much larger than Wooper). I've attached a picture of him from today after feeding, but I apologize for its quality - my camera isn't the greatest.

Again, any advice would be greatly appreciated! Thank you very much!

First up - get rid of the sand until he is a bit bigger. Sand to a small axie is the same as rocks to a bigger one - it could cause impaction. Sand is recommended from about 5". You shouldn't fridge an axie so small, but cool clean water is always best.

There are 2 possibilities here:
1. He's got an air bubble in his stomach. It will pass and he'll get over it.
2. He's a floater - a mutation that means he can breathe in air but can't breathe out again.

Was he floating before you moved him? If this is a new development it's probably temporary. Move him to a tub where you can keep the water level low - maybe a couple of inches - so he can stay down. Change the water daily, keep feeding him and wait until it passes.

Thank you so so much for the advice!

I have moved him into smaller glass container, sitting in a larger tupperware one with a little bit of ice beside it and a towel over top to help keep the water cool. He seems to enjoy being able to stand again!

You mentioned he may be a floater? If that's the case, is there anything else I should be doing for him in the future?

Finally, I've taken all of the sand out of the tank. No substrate now, just a few large, flat rocks he'll be able to sit on if he likes once he's back in there.

Thank you once again!