Nae, no heater- the waterfall action keeps res temps at ambient air temp, and the pump warms the water. Don't fret over my old system, it was experimental and its major failing was the crappy grow room I used to inhabit. Had air temps been under control (ie, not in a drafty pathetic attic), it would have been more predictable.

You risk salt problems in a soil environement when you use chemical ferts and don't water heavily enough.
In a hydro environment, it's an issue of finding the balance between adding WATER and adding FERTILIZER to the res between complete res changes. If you are not running your system with an EC meter, you are flying blind and in danger of getting the balance wrong. I wish I'd taken some pics of the scale all over my system before I cleaned it off, lol... I was running tap water with NO EC meter hahaha!
Anyone growing hydro seriously will check the conductivity of the solution before adding anything to the res, every time.