It's idea to start your own thread (left hand side, above the threads will say "start new thread" or some such. People won't respond as much here in another's thread that is a few months old.. I'll go ahead and answer here, if more people do then can stick with it, otherwise, try creating your own to get more people to help. Some people will recognize this thread and not come back as they had already previously helped. =)
It could be a few things though- the temp needs to be 78-82*, and no, you really shouldn't rely on the sun or a lamp as a heat source because at the end of the day, the sun goes down and the temp drops. Fluctuating the temp will stress him out and could easily cause him to go into shock. They need a stable, warm temp to thrive.
The cooler water would fit both the lethargy and the not eating as their metabolism is lowered greatly by cooler water.
Depends on whether your 1 gal is filtered. If it's filtered you will want to be doing 1 50% per week, siphoning the substrate. If it's unfiltered you will want to be doing 1 50% and 1 100% per week to remove the waste.
You say no additives, are you meaning you don't add in any water conditioner to the tap water when you do water changes? If not, that will end up killing him, and could also explain his actions. Need to use a water conditioner to remove the chlorine and other toxic metals found in tap water- dosing for the full volume of the tank each water change, including the partial.