hiya i am having the same issues with ph changing 3-4 times a day with my aero setup, palerider also ahd these issues with his hydro setup but found when he switched to a bigger tank/setup the ph stabilised pretty much, could be the answer.

Also, you do need a heater in your res, i think the optimum temp is around 22deg celcius, but i would double check that, i'm sure that is what i'm aiming for mine to be. you can pick them up from aquarium shops for around £15, (around $30 i think, maybe less over there), mine cost me £12 for one for a 200litre tank,(200watts) so not too bad. hope it helps