axolotl has constant arched back, can't figure out what's wrong?

[atlas04]

Before I go in depth about what's going on, here's the water parameters and water temperature!
Water temperature always stays between 64 degrees to 68 degrees, I keep my room constantly cold and so the temperature stays pretty still unless it's a hotter day (which we haven't had in awhile).
Ammonia 0 ppm, Nitrites 0 ppm, Nitrates 5 ppm, PH 7.4, and our tank is properly cycled.
He has two hides, fine sand, and a bunch of fake plants to keep the tank cluttered but not too cluttered. (I know real plants are preferred but I don't know how well those stay alive with fine sand?) And our tank is 20 gallon (we plan on upgrading in the new year.) Okay now on to what's going on:

My boy opal has had an arched back for the past few months. I can't narrow down if this is because he's stressed, or somehow got injured and I didn't notice. He eats' 3-4 earthworms every 3 days, he eats just fine. I've tried everything that I could to reduce stress to SEE if it was a stress-induced problem but nothing seems to fix it. He also has a floating problem, where he'll seem to float for days and then suddenly is fine on the ground. He always seems a bit distressed when floating, and I feel bad seeing him like that. Any tips would be so appreciated.
(Oh and I've tried tubbing him to see if it was impaction/constipation but it didn't seem to be either of those.)

 

[wolfen]

ideal temperatures for axolotls is 59°f - 64°f try to reduce temperature as much as possible (colder being better).
arched back can be a sign of stress.
make sure there is an air-stone in the tank (finer the bubbles the better) this is to reduce surface gulping.
try to feed a balanced diet annelids (worms) are fine but don't contain all the vitamins required, include shrimp, mysis, krill, fish etc..