Hi hippo,
Your axie can float for many reasons. Floating is a non specific sign that can be normal (physiological) or a sign of illness and stress (pathologicial).
1) High water temperature.
If your water temperature goes beyond 22 degree celsius for a period of time, one of the first signs of heat stress is floating. You should aim for a stable 16-18 degree celsius. Therefore check your water temperature.
2) Poor water chemistry
You want to aim for 0 ammonia, nitrites and <60 nitrates. Anything higher can result in toxicity/chemical trauma that can cause the axie to float. Check yoour water parameters.
3) Stressors
High water currents, other agressive tank mates can cause stress and thus floating. Check that there are non of these around.
4) Constipation/impaction
There shouldn't be gulp sized gravel, pebbles, marbles or beads in your tank. An axie which ingests one and has an impacted gastrointestinal tract can float due to build up of gases as well as generic signs of illness and stress. There shouldnt be gravel in the tank. Also feed a large variety of food types to prevent constipation.
5) The need to poop or 'burp'.
Gases in your axies intestinal tract can cause it to be more bouyant than normal. They are harmless and will be expelled through poop or 'burping' at which they would then sink back down.
6) Illness
Check that there are no other signs of illness or stress such as inappetance, mucus patches on skin, growths etc. These will stress the axie and cause it to float. They would require treatment.
Floating itself is actually stressful so if the axie only floats for short periods i would not be overly concerned but if it spends the good half of the day floating, something is not quite right and should be investigated.
Cheers