Water stored for years in water butts

Hi all,

On our plot, we have an opaque blue water barrel and black cylindrical water tank (not sure what it's actually called). They've both been full of water since we got the plot last summer, and presumably for some time before that as they weren't attached to a water collection system at the time and the plot hadn't been in use during 2019 before we took it on. I can't really see inside the blue barrel with any great clarity, but I get the impression their might be green algae or similar on the surface of the water. They are both quite full and the contain what I'm guessing is hundreds of liters of water.

I'm wondering if we should use the water in these old storage systems, try to empty them and clean them, or treat the water in some fashion. They have sunk down so that the taps are hard to access, but I can try to do so once I know what I'm looking for in the dispensed water.

Thanks!