"Oren" wrote

Window screen keeps the mosquitos out, but "no see ums" have no
respect for screen

LOL! I thought I would knock myself out a few times, trying to slap
'em off. Little '*******s' hurt..

Snicker, I feel your pain.

To the origional poster, I'd use a flexible plastic/vinyl window screening
or that nylon net you already have. I'd also add a light layer of some sort
of oil to the water top to prevent most bugs from being able to germinate in
there. In my case, it would be something food safe 'just in case kids or
pets' got into it. That of course assumes it's a cistern for grey water
use, not showers or drinking. (Plant irrigation etc, old cooking oil is
fine!).