Here down in FL inside the greenhouses, we use large propane heaters which heat the air. Now it isnt the most efficient way to do it, but it is among the easiest ways. Other ways which I have toyed with in the past is using a hot water heater/heat exchange system. Fairly simple design, but can cost more to install....efficiency wise it is far superior. Which way you go depends on how the tanks are setup (recirc vs independent). Of course there is always the traditional bar-type heater, which will work well if your tanks are all on a recirc system as you can get them at almost any size you need (from 1kW to 20kW+).