That Axolotl has not eaten properly for a very long time (weeks, maybe months) it has no fat reserves – probably due to improper care before you received it. At 20 degrees Celsius it’s metabolism will be high and it won’t last long. Now that you are seeing other signs of illness e.g. curled gills, skin looks bad etc it’s likely the animal no longer has the energy to fight disease – it may already be too sick to save.
If you can cool it down to slow it’s deterioration and get it to feed on some high quality food (earthworms, waxworms) items it may pull through. The prognosis doesn’t look good in my opinion.