You could ask liz2 about putting a heating pad underneath the bowl. I did that for one of mine for awhile. That's pretty cold for them. Hope you can get the temp worked out.
Just thought of something Laura...if he is still here..could you not put him closer to your new beauties it might perk him up?
I have got my latest rescue in unheated water..he seems fine...got no access to a plug at the mo and no heater anyway. better than being eaten in a large heated tank
It would have to be C because if it was F it would be frozen solid! 21C is just under 70F.
I've kept a betta in those temps before and they are not as active but it doesn't seem to actually hurt them. If it is a sensitive HM strain like melano or something, you might want to reconsider, but if it is a VT, which seem to be hardier, it'll probably be fine.
ok this might sound like a daft suggestion but if you don't have a heating pad then what about two rubber hot water bottles try attatching them somehow with string or summat to either end of the tank???