Temperature getting high...

Yep, lighting can contribute to heat problems. Is there a cover on the tank? If so, this is also contributing, since it reduces evaporation--the primary cooling for most tanks. Opening the top up to allow some air circulation will help. You can also reduce the time the lights are on, unless this is a planted tank. Fish only setups are fine with the ambient lighting (most rooms--if you have to turn the light on to walk across the room, it won't be enough for the fish either). If you don't already have one, I'd pick up a timer. All my lighted tanks run on timers, so the lights are on for a) the correct length of time for both the needs of the plants and my viewing time, b) a consistant cycle.

it could be the lights. is there any way of raising the lights off of the tank? if not maybe don't have them on as long. Or you could set your heater lower and see how high it gets.

I'd just like to note that 84 degrees isn't THAT warm. It isn't a problem in and of itself, i guess the only problem is controlling it. The lighting is probably a cause, as others have been pointing out, but IME i have 110 watts of CF lighting on my 46 gallon and that stays at a constant 79, and 28 watts of CF on a 10 gallon Q. tank, and that never fluctuates from around 80 - and those bulbs get HOT. it could just be that the room is warm?

Just a quick thought--have you seen the heater coming on? If not, turning it down won't have any positive effect. But, if it's set really low and something happens to cool the tank suddenly (ie, an open door or window), it won't be able to keep the tank from getting too cold. If it is coming on despite being set to a lower temp, you might want to replace it--a faulty heater is dangerous, both for the fish and for you.

I haven't had good luck with Rena heaters. I had one that behaved just as you describe and I jiggered it around trying to get it to lower the temp. Then one day I came home and found the tank temp at 95 with the heater stuck in the 'on' position.

No more Renas for me...

Jim