Well you MUST maintain proper temps or the frogs will eventually become ill and die. Their Kidneys and Lymphearts will suffer damage. You will not be able to place the space heater pointed at the tanks or their will be to much heat buildup inside the tank. Try using a heat lamp and see if that helps, but you might want an entirely metal lid as well.

Hopefully you can solve this problem quickly. They must have temps from 78° to 82° to remain healthy. If kept at anything lower for extended periods their will be health problem.