I didn't realize those Solana's had glass tops...
As roka64 mentioned, you're going to want to take that glass top off, or least open it up, to allow a small fan to blow across the top of the water. You don't need anything too big - just a little 6" clip on fan will do wonders. An open top tank will also allow for better oxygen exchange with the water and keep you pH levels a little higher... if you're having issues with that.
I'd really recommend having a heater in there. Just make sure it's set properly and don't rely on the markings on the heater to be too accurate. You might have to set it at 82 to get it to be 78, or vice versa. That way your tank won't at least swing to the low side of things. If the tank is running too warm, then it just won't turn on. At least with a heater in there, you won't be one of the folks that post around here each October, complaining that they lost a bunch of livestock because their tank got too cold and they forgot to put their heater in after summer was over!