It's tough to pinpoint heating requirements since there are so many variables, but I'll take a stab at it. As others have mentioned, insulation is key. You should definitely have covers/lids, otherwise you will lose a ton of heat out the top of the tank. I would also use rigid insulation underneath, on the sides and the back panel of the tank. I am using this stuff from Home Depot:
http://www.homedepot.com/h_d1/N-5yc...splay?catalogId=10053&langId=-1&keyword=rigid insulation&storeId=10051
Play around with this calculator:
http://www.hamzasreef.com/Contents/Calculators/HeaterChillerSizing.php
I have found that it's fairly accurate... although maybe a little bit aggressive (I think it actually overstates the wattage required by a little bit). If you play around with the calculator, you'll see that acrylic tanks retain heat much better than glass, insulating the tank makes a huge difference and that the wattage required increases dramatically the colder the room temps get.
Depending on how large your garage is, I think you will find that having that much warm water will actually raise the ambient air temperature (if your garage is insulated)...this is something the calculator does not take into consideration. So insulating your garage will also help keep the heat in- obviously, the heaters will cycle on much less frequently if the air temps around the tank are kept higher.
You're in a more severe climate than me, but I'll share my stats with you to give you an idea of how much juice it takes to heat my tank. The tank is located in a fully insulated, but unheated room detached from my house (think shed in the backyard). The average temps in my area are Lows of 39F and highs of 60F for the month of February (it's slightly colder in Dec and Jan).
My tank: 770G
Wattage: 1,800
Average room temp:~60F
Monthly KWHrs used to heat tank to 78F: 200KWHrs
My electric rates are $0.30 per KWHr, so multiply 200KWHrs by $0.30 to get $60 to heat the tank in February.
It gets a lot uglier (more expensive) if your air temps are drastically different than your desired water temps. I'm only raising the water 18F over the average air temperature and I'm still using 200KWHRs per month....if you are looking at heating water to 78 or 80 when the air temps are at or near freezing, that's going to require quite a bit of electricity. But I pay some of the highest rates in the nation, so if you are lucky and are only paying $0.10 per KWhr, you may have a monthly cost similar to mine even though you might be using 3 times as much electricty.
I would say that if your rates are closer to the national aver of $0.11 per KWHr, don't expect to have to pay anything less than $60 to $70 per month (minimum) to heat the tank in the winter. If you aren't ok with dropping that much money to heat the tank (I'm guessing it would actually cost you a bit more than that), I wouldn't buy fish that require tropical temps.
As an alternative, like Lopaka mentioned, heat exchanger off of your hot water heater would be more efficient (typically it's cheaper to heat via gas than electricity).
PS- Do I win an award for longest post ever on MFK?
