loland circulation help.

[Plant Heathen]

I just set up a lowland tank and in the proses of working the kinks out. It's an Exo-Tera 18x18x24. I have a 40mm PC fan in there and was wondering if you lowland guys/girls think that's big enuff. I have an 80mm fan I could put in there I'm just worried it will dry out the tank to fast. The humidity is at 80-90%. I'm using a small fish tank heater to heat the tank. Any tips or trick are welcome. I'm not a big fan of the heater but its the only thing I could think of to heat it.