Illustration above of a settling chamber or
other gravity fed filter systems such as niche skimmers
plumbing a retro drain to a niche skimmer, it is not recommended
that you use the "alternate feed pipe route" shown
above, rather use the below surface water level route shown.
For this application you would need two bulkhead fittings. One
to pass through the pond liner wall and one to pass through the
skimmer wall into the "dirty side" of the niche skimmer.
You should also provide an isolation knife valve and swing check
valve as shown.
When plumbing a settling chamber or a niche skimmer, always
use large pipe. General sizing guidelines are:
2" pipe from drain into gravity feed unit for ponds up
to 750 gallons
3" pipe for ponds up to 3000 gallons with pipe lengh
no greater than 15'
4" pipe for ponds larger than 3000 gallons.