Heating 180 gal tank with coils/cords?

[tropterrarium]

I'm fiddling with my new 180 gal (36"Lx20"Wx60"H) warm terrarium. I first thought I heat it with an immersion water heater connected to the water feature and blowing air over the warm water. This raises the humidity too much.

So I've been thinking of using heating coil/cord, place it by the intake of a fan (hidden in card board tunnel) and let the fan blow warm air around the terrarium. The heating coil will be turned on/off with a thermostat (HerpKeeper system).

How much wattage will I need to raise the temperature 5-10F above ambient? Standard wattages are 12, 15 and 25 watts, which seems a bit low for my application. How hot are those coils getting?

I've also been thinking of using an immersion water heater with regulated high temperature (such as the Rena or Cobalt Aquatics Neo-Therm), and placing that next to a fan. Those come in higher wattages (to about 200 W, which may be a bit too much.). Heat capacity of air is much lower, but by blowing air over it, it may do the trick. Has anyone tried that? Will certainly void any warranty, but may be worth a 50-bucks experiment.