Fish tank water heater

I am looking at building a solar water heater to heat up 3 tilapia fish tanks I have. We are in the tropics and it's warm out every day, but not enough to keep the temp at optimal of around 80 to 85 F. So I am thinking of using solar heating of water pipes to do the trick.

I am planning on making it small, I mean I don't want to cook them but keep them warm. Plus having a shutoff system in case it gets too much during hot days.

I am planning on using copper pipes for my system since copper is cheap down here and have the water returned and circulated using a thermo siphon system, so no electricity is used.

But I am having trouble with my theory and want to make sure it's all sound before I build.

For the thermal siphon to work, does my solar collector have to be lower than the water level or will the heat suck the water up a pipe? I would really like to put my collector above my tanks for optimal sun light, and shoot tubs down into my tanks for the inlet of water. But I am having a hard time finding out if the heat generated will suck water UP the pipes or if my collector has to be below the water level to get it started?

Thanks for any info you can give me.