Where is the dying part? Sponge or zoanthids?
Zoanthids can be feed by the larger food, like frozen Reef Plankton or dried Cyclop eeze. For a sponge, as I had read, the dissolved organics and the live phytoplankton should work.
I have this sponge, for a short time too. Had die-off of the sponge at the base (too much strain, resisting flow). Removed the dead part, inserted base in the hole in the rock added some superglue gel.
Had die-off on contact with hard to reach and eliminate aiptasia. Nothing I could do about it, but the main body of the sponge still unaffected.
Zoanthids are open and eating, as long as the food is present in the water (the tank is fed several times a a day).
High to medium flow. No physical contact with other corals.
Sorry, but it's all I have.