This is a bit late.
But my guess it that there is enough sun all year to set up a solar heating system. It can be as simple as a few meters of black hose lying in the sun, circulating to a similar setup inside.
Do a search for DIY solar, and you might get some ideas.
If the temperature drops a few degrees during nighttime, that will probarbly be fine with most fishes, not to mention that water works great as a heatsink.