If you're going to use a filter it is best that you baffle it so that the water is close to motionless. Here
is a step by step guide on how to baffle a filter of you can also do what other members have done by using Styrofoam cups, cut-to-fit filter media, or even aquarium sponges.
Also most heaters have a safety mechanism that prevents it from turning on when the temperature that it is set to is reached, so you can turn on your heater with no worries. If it is adjustable be sure that you have it set to at least 78 degrees and below 85.