It's not mandatory in a FWOLR....but it will make life SO much easier if you use one.
I had wicked cyano
problems in my 40 FOWLR
which is only lightly stocked. I used a (don't laugh) Prizm skimmer.
When I upgraded last week to a CPR bak-pak, the cyano
suddenly started to diminish...and now a week later I have very little cyano
. Why? because my skimmer's pulling out all the crap that the Prizm didn't...so it's no longer breaking down into phosphates.