Silica too low?

Also, you brought up another interesting point about sponges not using silica directly...

Are you suggesting that their ingestion of diatoms is solely their source of silica?

 

I'm just saying that they definitely eat diatoms, and that diatoms definitely use silica, so there's another explanation for sponge success when silica is dosed that doesn't rely on what is essentially a wild speculation that they use silica directly. One theory is tested, while the other has absolutely no scientific backing and a bit of anecdotal evidence (that can still be explained by the first theory just as well), AFAIK.

I would probably keep a refugium with a few smaller sponges rather than a gigantic one. Something like a display fuge, but with decent flow so the sponges get plenty of contact.