What you need

• 1 x glass tank. Size = 1.5 x Water capacity of your frog tank.
• 1 x pump.
• 1 x Hose enough to go from your tank to the sump tank and back again.

The Design
The basic design of this cooler is simple. As the 'rain' is only on for 4 hours a day, half the water sits without a heater for 20 hours thus losing a few degrees and becoming monsoonal rain. To operate this cooler requires a small pump in the sump tank and a siphon from the bottom of the frog tank to the sump tank, it is important to make sure the rate at which the pump pumps water round is the same as the rate in which the siphon siphons the water down, otherwise you may end up with a flood. Another important point is the siphon must only extend halfway into the water otherwise if for any reason the pump fails you may siphon all the water out of the frog tank.

Assuming you have this set up correctly when you want the 'rain to come on you can simply have a block in the bottom of the siphon which you remove at the same time as turning on the pump, it may also be wise to turn off the heater as the whole point was to cool the water. This is just a basic design and I would be happy to hear from people who have altered the design or added automation to the various parts.