Having a flow through fry system set on the top of a heated 20 will also work. That is pretty elaborate though.

In such a system a small powerhead pushes water up to a bowl, sitting in a tray above the tank. That bowl acts as a central reservoir. That bowl has a piece of stainless steel screening - fine mesh - melted into the side of said bowl or container as an overflow. Small siphon tubes are run from that bowl to the surrounding fry containers. The fry containers sit in a tray - maybe one of those plastic, ventilated, trays from the garden center when one buys all the spring flowers and veggies. Each fry container, which could also be covered, has a stainless steel mesh melted into the upper side of the container as an outflow. The water drains back into the 20 to be filtered and heated and pushed back to the central reservoir. There is so much evaporation in the winter from such a tank, a little R.O. water needs to be added or evaporation of tap water used to top off the system causes the whole thing to get incredibly hard.

When we had that aquarium club where I (allegedly) taught, a friend active in the IBC gave us several great Bettas. Classrooms cool off dramatically after hours. The situation was temporarilly solved by floating the Bettas in several jars in a ten gallon tank.

Ironically the warehouse of that contractor recently caught fire. Makes you wonder how it was heated. ;)

I have spilled water on the one warming the fishroom without incident - other than having to towel up the mess.

The one in our family room isn't working. Perhaps the best idea is to take in it - to whom I don't know off hand - and then lending it to you for the winter.

One water proof unit in a room (God help us, never use a room warmer with exposed heating wires!), a closed door, maybe that plastic stuff over the windows, might well be less expensive than 11 small tank heaters. It is a four foot unit and would have to be on the floor at munchkin level though.