Illness/Sickness: Ningyo Handstands

After two weeks of just joy with our new family members, there's allready a problem . Ningyo does handstands. He's been doing this for a couple of hours allready. Its like he has gas in his belly. He can still walk on the bottom (and does not float around up in the tank !!) and he does stay on the bottom of the tank, but his rear end is up. What should i do?
For the rest, he is behaving normal and eating very well!

I did a lot of reading before getting axolotls, so I think I do everything right.
Further information:
- Two axolotls in a tank of 90 liter (long side:80 cm). I will upgrade this in thye future, but for 2 axolotls of 11 cm this must still be okay?
- No gravel!!! We use river sand.
- Temperature is between 19-20°c
- Food: Bloodworms and ocasionally an earthworm. We feed every day.
- Water stats are okay: nitrate: 0, nitrite: 0 and ammonia also 0
- Filter current is not too strong and it blows directly against the glass.
- Pachua (the other one) is fine

Wat should I do??? I want him to grow up a big fat healthy axolotl!!!

It sounds like constipation. Wait a few days and see if it goes away. If not, you may try putting him in the refrigerator for a few days. This often encourages them to pass waste.