Hi Mido,

In fact there is a way of reducing the heat in the water tanks fixed on top of the roof.

There is a kind of material which you can find in hardware shops. (Najma), Just explain the shop keeper that you need the material or sheet for this purpose.

They will guide you. May be it's kind of Aluminum Foil which you cover the entire water tank in the roof.

Believe me, it works for a certain extent. (Sorry for the improper information)