I have surface scum on all of my FW
tanks. Some people say it does no harm, but I believe it affects the oxygen exchange at the surface. I know it is caused by something in my public water, because when I set up my 150G, it was there when I first filled up the tank. The only way I control it is by hooking up an AC
surface skimmer on the input hose of one XP canister in each tank.
I like your idea of a HOB
pre-filter that then hooks up to a bio-media only canister. I hate cleaning my canisters. For each canister cleaning I fill three 5G buckets with tank water to clean and keep submerged the bio-media baskets (avoids killing off too much BB) and filter sponges; what a mess! Because it is such a pain, I let the waste sit far too long in the canister before I clean it out.
I also have one HOB
on each tank as a backup (especially during a power outage). I love the ease of cleaning those.
It would be great if a canister manufacturer read this thread and released a HOB
/canister combo; where you have easy filter cleaning, water bypass back into the tank if you let the filter media get too dirty, and no need to clean the bio-media sealed in the canister.