You absolutely need a heater. you need to keep the temp from chaning too much. the aboslute temp isn't that important. some people run 76deg, some run 84. I shoot for 79.
You want to set the heater so that it keeps the temp about the same as the MH
's keep it in the day. so let's say you get to 82 in the day, you should set it for 81.5 or so.
Fans work wonders and are a lot cheaper than chillers, if you find that the upper temp is too hot. See what happens first. An open top tank/sump helps a lot too.