What a great way to start seeds in a greenhouse with a heated aquarium! They make 100 watt and 50 watt submercible heaters for 10 to 50 gallon aquariums! besides the heater having a bubble air insert during the heat of the day will also raise the temp in the water. I have 3 clear thick plastic shower curtains closing off the heat chamber from the rest of the greenhouse with a 1 inch styrofoam insulation at the back of the chamber! This works great!
The water will actually cool the chamber when its too hot by absorbing heat! So many advantages of the heated water. Also the aquarium will give moisture to the chamber! You can set the heater so it goes on only after dark or in very cold weather. Some of the better heaters can also be set at even 85 degrees. Most of the heaters are set for 75 to 78 degrees! Any other ideas let me know?