Temp Regulation Question

[tttaylorrr]

It is a 40 gallon tank with an 100w bulb. The box read that it was for 30-40 gallon tanks.

that sounds right. i'd still get a little $10 lamp dimmer (rheostat) for it (all heat sources must be regulated) and see if you can dial it in. also, if your enclosure is a bit tall then of course the heat will have a harder time reaching the floor, which you can compensate for by using a higher wattage bulb.

it's all like a mini science experiment. testing, testing, failing, and more testing.

EDIT as an anecdote: i used a 100W CHE on a rheostat for my 20gal (24x12x12), but i only turned it up the tiiiiiiiniest bit to reach 77° F.