What I usually do to check and make sure my axies are not constipated, have air bubbles, etc. is while the axis is floating, I gently slide my hand underneath it and lift it a bit out of the water. Usually he/she will swim back to the bottom of the tank, and if the axolotl floats back up or struggles to stay down, it means it is constipated/impacted. The swimming and pinkness in the tail is most likely from your nitrate spike, but now that you've fixed your water it should go away. If the pinkness gets any worse or looked raw/irritated you could try giving your axolotl a tea bath or put almond leaves in his water. The air bubbler shouldn't be causing any problems, so I think it would be fine if you put it back into your tank. Anyway, do the constipation/impaction test and let me know how it goes. Although If your axolotl has been floating for more than 24 hours I would definitely fridge him.