i dont have a heater in my tank but it stays plenty warm without one seeing as how i'm in georgia. i have a pretty strong plant light too though so that contributes a lot. they are pretty hardy though as far as fish go. he was probably sick already when you got him since he only lasted a week. that is one good thing about petsmart--they have a 2 week guarantee on their fish--and i have had to use it a few times. there's not a whole lot you can do if you happen to get a weak or sick one from the batch.

your sig is a little long, but definitely not the worst i've seen. you could separate your animals with commas or periods if you want.