They used to be called cryptic zones, not sure if that still is the terminology or if they've been debunked. But I remember Steve Tyree swore by them. A lot of folks here treat sponges like nuisance algae, I'm fairly neutral to them since I don't have any issues in my display. Maybe @Thales has an opinion on this!
But lower flow areas or tightly packed things could potentially trap other things like biofilms, etc. and start to impede flow into the return chamber.
Cryptic Zones in the Refugium | Refugium Education | AlgaeBarn
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Reef Farmers New Book
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But lower flow areas or tightly packed things could potentially trap other things like biofilms, etc. and start to impede flow into the return chamber.