Not sure if there is such a thing as a more or less "economical" heater, simple physics says it is going to wake X watts to heat Y litres of water to Z degrees above ambient temperature, regardless of how it is provided. The two ways you can save money is through insulation, and finding more cost-effective means of providing the energy required to warm the water (eg gas heater, inverter/heatpump, log-burner?, etc). Also, if you're running a sump use submersible pumps instead of external, it might not be a lot but it is still "free" heat going into your tank water rather than disappearing into the atmosphere.
I use 5 300W aquarium heaters running on a custom controller/thermostat with a 1C tolerance (ie on at 26C off at 27C) on my 2000L set up, the sump [which is in the garage] is insulated with polystyrene, and the tank is in a well insulated room that is heated to 22C with a heatpump over winter to minimise the loss of heat. Once up to temperature the heaters hardly come on.
I use 5 300W aquarium heaters running on a custom controller/thermostat with a 1C tolerance (ie on at 26C off at 27C) on my 2000L set up, the sump [which is in the garage] is insulated with polystyrene, and the tank is in a well insulated room that is heated to 22C with a heatpump over winter to minimise the loss of heat. Once up to temperature the heaters hardly come on.
