Hi on388 and welcome to the forum. I feel your pain. I have the same problem. As many tanks as I want in the garage but out of room in the house. One way is to get a heater rated higher than your tank, so try using a 200w heater. The only danger there is if the heater malfunctions, it will heat up the water very fast and you could lose your fish before you even know it. Place the heater as close to the filter outflow as possible so the warm water circulates. Since it's a 20gH, you might also consider putting an airstone in to circulate the water even more. See if any of this helps.
If you can, try repeating the test using indoor room-temp water and see if the heater still struggles to keep it at a decent level. It may have struggled because the water was already cold to begin with.
And if all else fails, do what I did: put a goldfish in it. :P