Just another thought.... when I first got my female axie, it was July in Texas and about 105 degrees everyday. I had my AC on full blast, but I still had a difficult time keeping the water temp under 72 degrees. When the temperature crept up over 72 degrees, she would float like a bobber. In her case, the buoyancy was caused by her tank water being too warm. I now have mini-chillers and keep the water at a cool 64 degrees.
Here is some information from the FAQ section of
Axolotls: The Fascinating Mexican Axolotl and the Tiger Salamander. The question was regarding why an axolotl would start floating.
"Often, some stress will lead to a small bacterial infection in the gut. This would lead to a gas build up, and the floating. It is possible that the axolotl swallowed a large amount of air into its digestive system too. Warm temperatures (over 20 °C / 68 °F) don't help. Anything over 25 °C / 77 °F is far too warm for axolotls - just in case you didn't know.
As to how to solve the problem, axolotls find it stressful when they can't touch the bottom. I suggest you lower the water level so that the axolotl is touching the bottom, but still submerged. The fact that it can touch the bottom should help to relieve the stress and hence help it to recover. If you keep it at temperatures over 20 °C / 68 °F it will probably make it harder for the axolotl's gut to keep up with any bacterial activity, so try and keep it cooler. As long as it eats, that's a good sign."