GFO's and and Activated Alumina (phosphate sponges) both remove silica and are very efficienet at it, more so than removing PO4, especially the activated alumina type. We have some small issues with the activated alumina type, as they release Aluminum which irritates some corals at times.
Sponges growing and doing well are the signs of a good tank not bad tank. Steve Tyrre wrote a book on sponges and ascidian and their abiltiy at filtering water
The Environmental Gradient
Crypitc Sponges and Sea Squirt Filtration Methods
Volum 1 of the Captive Manitenance
Advancted Techniques CMAT Series
Sponges, Phylum Porifera
Aquarium Invertebrates
Some things to read;
Aluminum and aluminum-based phosphate binders
Chemistry and the Aquarium Iron Oxide Hydroxide (GFO) Phosphate Binders by Randy Holmes-Farley - Reefkeeping.com
Silica in a Reef Tank
Advanced Aquarist Feature Article
Phosphorus: Algae's Best Friend
Chemistry and the Aquarium