I think my betta may be dying

Question
My crownatail betta was in way too small of a tank (like maybe a half gallon?) that I bought cheap at a pet store for my dorm room, not knowing it was too small of a tank- This past week I bought him a larger one gallon tank with a filter, light, and a plant etc... I let the tank run for about a day and a half and then moved him into it last night. I'm not sure if the move was too stressful or if he's sick, but since I put him in the tank he has just sat on the bottom gradually getting more and more lathargic. He is almost just floating on his side on the bottom now and won't swim around even if I put a mirror or anything up to the glass. He doesn't appear to have fin rot, ich or any other noticeable illness. He's just become totally unresponsive and inactive. I think the water is too cold however I won't be able to get a heater for at least a few days and I wasn't sure if there was anything I could do in the meantime or if there was any way to be sure nothing else is wrong. I don't know a lot about bettas and it's probably some rookie mistake (definitely should have done more research before getting the fish) however I've really become attached to him and I'd like to do whatever I can to keep him as healthy and happy as I can! (considering I'm a college student in a dorm room...) Thanks for your help!

Answer
Hi Anne!

First I want to apologize for getting back to you so late. My email was temporarily out of service :(

Well congrats on upgrading your betta. You will have him much longer the bigger the tank he has and the more room he can move around in. There really isn't a heater on the market that is safe for a tank that small. I would suggest maybe a desk lamp to let mr betta get some rays and some heat from it as well. Make sure you don't get it too hot with the light. He is probably a bit stressed from the move. Big PH swings can do that too. In the mean time watch him closely for more signs of illness. If he is not better then you will know you need to figure out what is wrong so you can medicate your little friend. It is normal for them to be quite stressed from moving his home. But you have done what is best for him. I'm sure you already know to match the water temperature to make sure it is the same when doing water changes which you should be doing a lot of in a tank that small. Hope he gets to feeling better. Write me back if not and we'll see if we can figure out what is wrong. Good luck!