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.