Seal around the garage door to eliminate drafts and allow the garage to accumulate enough heat to raise the temperature a few degrees above ambient. Provide 1,000 watts of heating capacity and set the temp to 2-4 degrees above the actual desired temp. This will allow the tank to accumulate additional heat during the day (when it's warmer). I think you'll be ok.
Just a side note, does any one else find the temp on the heaters don't match the thermometer? I had 2 300watts on my basement tank set at 80 and the water was always 78, I tossed in another 300 watt and the water is still 78. So I assume that the heaters think the water is at 80.
Heaters are calibrated to be within three degrees of the actual temperature. Some brands/models are better than others.
Does the tank have a wet dry or sump? If it does and the pump is large, it will also help heat the water along with the heaters. 125 sounds like a lot of water until you consider the external temp and the insolation of the garage. A friend of mine has a 900+ gallon tank in his garage. He has a large wet dry with a very large pump that puts out a lot of heat. In addition he has six 300 watt stealth heaters in the sump. The garage has a built in heater, but he rarely needs to run it. An acrylic tank of this size/thickness holds temp for many hours longer than a glass 125 will.
I have a 90 gallon in my garage that gets down into the low 30's at times. I used aluminum "bubble-wrap" type insulation, mostly because I had a roll of it lying around. It stays warm with just a single 250W heater, but I'm thinking of adding a second just in case of failure. I do have a tight fitting glass top as well. On days when it gets really cold I sometimes throw a piece of foam board over the top of the tank to help retain a bit more heat.
If you have any HOB or external filtration be sure to insulate the filters and hoses, the rapidly moving water increases the heat loss considerably.
