I know I found some valuable info on keeping these guys a while a back from some people who kept them long term (if I recall, a couple of them had them for a couple of years), but I can't remember where I found it at the moment. I'll update if I remember where to find the info again.
As mentioned in the linked thread above, though, the NPS zoas growing in the sponge are parasitic, so you have to be careful to help keep the sponge growing as fast or faster than the zoas (not an easy task at this point). From everything I've heard, these sponges are substantially harder to care for than the average as a result. They die for most people within weeks or months of them being added to the tank.
I would guess you would see the best success with these guys by maintaining good nutrient levels and providing proper bacteria species as feed (though some people have reported success with phytoplankton, so some species of phyto - likely the smaller ones - may work as well), but - from the few sponges I've looked at the diets of in depth - the more important nutrient levels may be the organic nutrient levels (which, to my knowledge, we can't really test for with hobbyist test kits). Basically, proper flow, proper (not normally measurable) nutrient levels, and proper feeding (with the proper feeding possibly involving both phyto, bacteria, and dissolved and particulate organic nutrients) seem to be the key to keeping most NPS sponges from what I've read.
