Heating the room is usually much less expensive than heating individual tanks is the space that the tanks are in is not much larger than the immediate area that contain the aquariums and that area is well insulated against the loss of heat. It takes a lot of energy to make water increase temperature. If you heat the room the water is surrounded by warm air and will absorb heat until the air and water are at an equilibrium. If the air is cooler than the water it will take heat away from the water. (heat always transfers from warmer to cooler) If you heat the tank with heaters and have cool air outside the tank, the heat will not stay in in the water. Just say no to heaters!
Looking at the money, if a piece of equipment runs 24 hours a day for a year, the cost in dollars (in most of the USA... more expensive in some states like CA) will be approximately equal to the wattage of the heater. So a 250 watt heater woudl cost $250 to operate for a year. But heaters do not stay on all year... but what if they stay on 6 hours a day? That is 1/4 of a year, so the cost would be $62.50/year for that 250 watt heater. If you have a fish room with 50 heaters.... it adds up pretty quickly!
