A thought came to mind about your continuing battle with algae. (Mine continues too, though the problem is not as bad as it was... I still need to further increase partial water changes.)

In the area(s) where the algae is, would there be a time or times in the day when the sunlight "sneaks" in? In mid-summer we get some odd angles. Now that the sun is "going south" I notice some afternoon illumination of our living room tanks on their SW corners.

If so in your home, perhaps pulling a shade, closing a drape or taping some paper over that outside of the tank would be useful. In the winter months, even a piece of black paper (NOT construction paper or some wrapping papers, which can run like crazy when wet) or foil or plastic could be used. Guppies look even better in front of dark backgrounds and a little heat might be absorbed by the black and transfered to the aquarium. I wouldn't use dark paper in the summer. ;)