Warmer Freshwater Aquarium Options

Question
I have recently set up a planted 30 gallon freshwater tank.  After running the tank for a week with only plants, substrate and water (filter and heater too) I put 4 zebra danios in it.  The tank is running hotter than I would expect (78-82 F).  The room is air conditioned (set at 75 F), I have kept the shades closed and put a fan near the tank.  I replaced the hood light, heater, and thermometer and have left the glass top open for a couple days.  All to no avail.  The fish have appeared sluggish since day 1 (about a week ago) and unfortunately 2 of them have died.  I'm not really prepared to drop hundreds of dollars on a chilling unit.  Can you tell me which fish are suitable for a community tank that just won't cool below 78?  I leave the heater set so there won't be a temperature plunge each evening.  I would love it if you can help me prevent future casualties!!

Answer
If the heat is the problem (I am not convinced it is) why not take the heater out all together?  
I don't have a heater in any of my tanks.  I have one tank that is 78 most of the time.  I have neons and a betta in it, they have been in it a long time, and are all doing well.
No doubt if you aks 10 hobbyist you will get 10 different answers, but mine is, remove the heater all together, a couple degrees of change will not hurt, water is not the same tempature all the time in the wild.
Good luck.