Well, people have used this type of set up for years to heat pools.
I have a few concerns though..
1- I think it might heat the water too much for an aquarium, it would be hard to regulate the temp of the heated water, based on the outside temperature, and the amount of available sunlight.
2- the cleat pipe will just provide light for some serious algae to grow.. I'd just leave it out, and run a UV if you feel the need.
3- the water in the tubing between the aquarium, and the roof is going to be way off temp, if its 60F outside that water is going to be COLD.
Maybe you can runa continious cycle of water through a seperate storage containe with the thermostat, and not pipe it directly from the roof to the tank, you could store the warm water, and if the temp drops too low, then click on the pump..
Just my Opinion