some tanks are others arnt.
I use 1 bulb to heat 2 tanks, ie the one its in and the one above. As said it creates a heat pad in the above tank so no need for another heat source.
In a bank in 3 large enclosures i only use 2 bulbs, one in the bottom enclosure, which creates a heat pad in the middle enclosure, one in the top enlosure that creates a heat pad on the top of the bank. i can then butt 2 x fish tanks over the heated area or 4 click clacks, and end up heating 5-7 tanks with 2 25 watt bulds over summer or 2 40 watt bulbs over winter. Too easy.
This also crates a nice warm area in each tank with the rest of the tanks areas being completey unheated to create a large temp gradaint.