Yerr, this lad knows all there is to be known about fish by the sounds of things...
Yes you could put a heater in, but what's the point when you can just leave water over night and forget about it without purchasing a heater.
Fluval make the best pumps in my opinion, they seem to work for us anyways we have 2, 1 either side.
One minute you are saying change water once a week, now it's 6 months, make up your mind...
Like I say, we have had our tank going for 20+ years so I think we may possibly know what we are talking about.
We run our tank exactly the same as Chris at plymouth discus and he knows everything there is to know.
All I know is, we can keep our Discus for years, and they are fish that have to be in perfect conditions.