I think there is a lot of detail that needs to be considered before taking or making advice on this...
First, insultate! spending a little money now on insulation can save you a lot of money down the road. Keep in the heat you pay for!
Using an electric space heater is not going to be any cheaper than using electric heaters to heat the water. But the electric heaters in the water make it much easier to control the temperature.
If you are using sumps or a central sump system then the pump or pumps that are submerged will heat the water... Other filters will also cause heat which will heat the air...
Humidity is going to be a concern in that room. I highly suggest running a dehumidifier. Dehumidifiers tend to generate a lot of heat as well.
There is a really good change that if the room is decently insulated, that the heat generated by the pumps, filters and dehumidifier will be all the heat you need. I would drop a heater or two in the sump set slightly lower than the desired temp as a "just in case" backup.