I did this about 40 years ago using a Hagen (now Fluval) 403 or a 303 with 2 intakes and 2 outlets. I sealed the uplift (now down push) between them and the powrfilter outlet tubes with baby bottle teats with the end cut off to form a seal worked well.So I saw a pretty effective idea to incorporate a reverse underwater filter. Use a canister filter return pipe, and put it into the tubes feeding the filter or filters and just control the flow by restricting the tubes with the excess spilling into the tank ? Opinion ?
Now I don't use UG filtration anymore but I do have sand ont he base of my aquarium.
Remember, water will travel along the least resistance and most water from the down tubes will exit close to it.


