If he's just new to the tank, it could be just that it'll take a few days to settle in, nothing to worry about. But really, being chilly could be it, also..
Can you possibly put him in the warmest part of the house/close to a heater? Wrapping the tank in towels at night when the temps tend to drop the most can help keep it warmer, also.
Things tend to be expensive in the shops here in Aus, though.. but I found I can order stuff quite cheaply on ebay and the like. I'm just careful to read the negative comments on the sellers.
If you do decide to get another heater (I personally would return the other one, as it didn't do what the shopkeeper claimed it would and all..and think of it this way - a good quality heater isn't a 'waste' of money, as much as an 'investment' for your fishy.. ;) ) consider an adjustable 30watt. I spend a bit more on Dymax mini-heaters, but I have never had one go wrong yet. Well worth the extra, and they go for half price or less on ebay sometimes. Anyways, 'adjustable' is the important part.
Anubias is a lovely plant, very slow growing though, so it eats less fish pee than say, java fern or moss.
I would quit using the salt, if you are changing the water enough, that is all the fish needs.
Well water isn't as good as regular tap water, either -- is that the only water available? There's some threads here somewhere on coping with well water, if so..