In message , Fredxx
writes
On 07/06/2021 08:46, Tim Lamb wrote:
snip
Can you insect-proof the top of the drain pipe?
I have lots of fine mesh plastic netting bought to protect garden
produce for tank vents. Gutter and down pipe not practical but I
assume insects lay their eggs in standing water.

A film of oil tends to stop that! It does mean the overflow must dip
below the surface and have a anti-syphon feature

Yes. I had forgotten that. Reduced surface tension so the wrigglies
can't hang there to breathe?

The plan is to use the water for garden/greenhouse irrigation so it may
empty in long dry spells.

--
Tim Lamb