Heating big tanks depends, if your room is insulated you can heat the room to 25C lets say (change it to f in google

) then the tank water will soon lift to 25C in a week or 2, and you can use 1 heater 500W or 1000W.
This way is best as glass cant hold heat at all, so when your air temp is lower then tank temp it will cause heat exchange between the glass results in your heaters heating the room also. if everything is at the right temp room wise this wont occur so the temp is going to hold.
if your rooms not insulated your looking at about 2-4 1000W heaters I would say to hold the right temp. I good idea is to use EPS insulation on each side of the tank and sump apart for the front, this will keep the hot water in the tank, it cheap and easy to use plus water proof.
as for getting the room to 25c or so your cheapest bet is a r/c unit but they are costly up front but cheaper to run then gas in the long run, otherwise your going to want a oil filled heater which are costly to run but cheap to buy.
just remember heating the room is only cheaper if its insulated, otherwise your trying to heat the world, it will also lower the amount of evaporation that occurs saving you water.
Look into solar heated hot water also if you can figure it out you can run a coil into your sump off a thermostat which will work better then a 1000w heater and is cheap to run but requires some knowledge.