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thrlride... 20X is really just a ballpark guide. Like lighting... it needs to be adjusted/finessed on a case by case basis for the specific animals you keep... and presumes you will keep a realistic and compatible group of organisms that can actually fare well under the standardized parameters you set.
That said... the 10-20X guide is for the individual tank (display or refugium). Thus... for Chaeto in a 20 gall 'fuge... a 400gph flow (20X) is not asking a lot. Its not enough flow in many cases (like when significant particulates enter from incoming water). In such cases where aquarists get misled to use the old slow-flow recs (under 10X)... it is no wonder the refugium becomes a cess-pool of nuisance algae.
Slow flow in most any refugium (other than settling chamber styles that are actively serviced) has been one of the single biggest tidbits of "mythinformation" promoted at large in recent years. Its patently bad husbandry IMO.
Anthony
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"If you give a man a fish, he eats for a day... but if you teach a man to fish, he eats for a lifetime."
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