From my perspective, what is in the tank is unimportant. Bugs will grow inside dark tanks, especially if they are allowed to become warm.
What is critical is what comes our of the taps.
We chlorinate all water added to the tanks and then filter via a 5um sediment cartridge followed by a 0.5um active carbon and silver cartridge. That process makes pretty much all water biologically safe.
Cheers,
Peter
OKA196 motorhome