OK, trying to sort through four pages of posts here.... Does this summarize what's going on?
- 2.5 gal tank, but currently floating in a cup
- 72 degrees F, but temp is currently being raised to (what?)
- No filter
- Air stone
- flat preset heater (but you're concerned he'll get burned on it, so it's under the gravel)... and you have a new 25W adjustable heater
- fasted for 7 days due to constipation and bloating
- water change acclimation rushed
- daily partial water changes to keep ammonia low (since he's in a cup)
- about 3 yrs old
If that's all correct, then:
I would keep him in the 2.5 gal tank with a lowered water level (or just enough water to cover the heater, if you need more than that). This will allow him to reach the surface easily, but will provide a larger amount of water overall so that ammonia won't build up so quickly. (Don't worry about him getting burned on it. It's only going to raise the water temp to about 79F. Our own body temps are 98.6F. So the heater isn't even as warm as we are.)
The preset heater may be fine for heating this small volume of water, or you can use the new adjustable heater.... But when raising the temp, raise it no faster than 1 degree per hour, with 5 degrees per day max. (I think Callistra mentioned this already.)
Is he still bloated? Has he pooped at all? How much Epsom salt did you have him in, and for how long?
How did you acclimate him after doing partial water changes?