I currently heat all my tanks through ambient room temp in the frog room. I control the temp through a combination of a space heater and heat cables running under the tanks. In the past I heated all my tanks with aquatic heaters and it works beautifully. Soil and plants will help keep the humidity up more. The temp of your tank will be affected by the ambient temp outside the tank as the heat radiating off the water fights the coolness of the vivarium glass. I did find that I had to set my Aquatic heaters a few degrees higher than I wanted to keep the Viv temp where I wanted.

Mantella are very sensative to cold temps. They do not handle dips below 68. We had winter lows hit us that were well below 0 for several weeks and overnight my space heater couldnt keep up and all my vivariums dipped into the mid 60's. I caught the dip at midnight and placed a 2nd space heater in the room and wrapped my shelves in blankets. All my other frogs are fine, but the next morning I found my group of 5 Ebenaui and they had not made it.

Another trick (I picked up from Lynn) is that I now have an Aquatic heater in my 5 gallon reservoir for my misting system. This removes in temperature dips that misting with room temp or cooler water might cause.