I have cubos as reserve water for my animals if our agriculture mains goes offline. I covered them with wooden frame and plywood. The water stays clear and fresh and I have been using this system for several years. I just drop it and refill a few times a year especially after summer to avoid legionella which breeds with sufficient heat. Tarps would work to I guess but they decay fast in the sun and wind damage. I prefer a more permanent measure. You could also bury them if your drawing with a pump anyway. Or go the whole hog and brick them up.

The Royal Navy kept its water barrels fresh with a tot of rum. Don't know the ratio but I am sure it could be fun figuring it out. ; )
Bruixot