Sounds like any filtration method that would do the job would be more trouble than it's worth, so I think I'll just draw water from below the surface and then bleach it - if it seems funky. Otherwise, use as is.
N.B. The tanks
I'm talking about are very small (like 2-3 gallons) water cooler type tanks with a spigot, so their "funkification" is not an issue (can be very easily taken on deck
and cleaned). And, anyway, with a little bleach, nothing is ever going to grow in them.