Axolotl refusing to eat and arched back

[Egplant]

I've had my axolotl for 4 years since rescuing him - he was kept in really bad conditions so had no gills or toes left when I got him and badly burned skin from high ammonia. He has since grown his toes back but his gills never recovered (just starting with this so people understand the lack of gills isn't due to current water conditions/ unrelated to current sickness ).

He has stopped eating recently and has a very arched back. He also appears to have a lump between his front legs. It was quite sudden, but there had been a bit of warm weather where the tank water got up to 19 degrees Celsius at some points. Water parameters are all normal other than that. he gets fed earthworms with the occasional piece of prawn or a wax worm and has never refused food prior to this ( sometimes takes it and spits it back out if he’s full, but never been uninterested)

After him not eating and being very lethargic for a week I put him in the fridge at 6/7 degrees as I was concerned about impaction / constipation (although he has never been kept on gravel as far as I'm aware). After him being in there for 3 days he showed no change apart from occasionally passing some small mucous like pieces. I have fully cleaned the tank, 100% water change and cooled the water down to 16 degrees while he was in the fridge, so slowly acclimated him back to the tank as I thought the fridge wasn't making a difference.

Now I'm worried maybe I should have left him in the fridge for longer?

Please help!! He must be around 11 years old now btw but I don't know definitely how old he was when I got him.

 

[wolfen]

arched back is another sign of stress similar to hooked tail (both caused by muscular response rather than voluntary), if the water parameters and chemistry are good then the stress and lack of appetite could be caused by the sudden development of the lump that was mentioned, if fluid filled adding 3.5g/l non-iodised salt to the water should help reduce it's size easing other symptoms.
although difficult to find a vets opinion would be the best option.

 

[Egplant]

arched back is another sign of stress similar to hooked tail (both caused by muscular response rather than voluntary), if the water parameters and chemistry are good then the stress and lack of appetite could be caused by the sudden development of the lump that was mentioned, if fluid filled adding 3.5g/l non-iodised salt to the water should help reduce it's size easing other symptoms.
although difficult to find a vets opinion would be the best option.

Do you know if there’s a way for me to tell at home if the lump has fluid in? Or if not, is there any harm in trying adding salt? I’m struggling to find any ‘exotic’ vets nearby, the one I did find told me they couldn’t advise on Axolotls!!

 

[wolfen]

it would be risky to try piercing the lump (a vet would have been able to scan it to find out of it was fluid or tissue) and could lead to infection although a bright light shone through the lump could be used instead (tissue mass will show up where as fluid is quite translucent, although may be difficult to hold a light and axolotl so an extra pair of hands would help).
3.5g/l salt is the same level as is used in 100% holtfreters and isn't high enough to be harmful.