Main challenge with tanks- whatever the type- are lizards and rats, which drown and die there. 99% of water tanks will have these vermines which normall access the tank through either the overflow pipe, unclosed covers or open down pipes.
A sure way to keep the water clean esp for masonry tanks is to have a collection pit before the water flows to the main tank to catch the vermines before they get to the tank-most often they will be dead!
"Things that matter most must never be at the mercy of things that matter least." Goethe