Seems like big sponges need a lot of food. I have lots of little sponges spouting out of rock. But when I ordered a Carribean red ball sponge about the size of a baseball, it got smaller and smaller everyday I owned it. Regular DT's phytoplankton didn't seem to be enough and it finally busted into pea-sized pieces. Now the pieces seem to be stable little sponges. If I had a nano-tank, I'd be in great shape! Also interesting is that each pea-sized piece has it's own communal zoanthids or whatever they are that encrusts the ball sponges.
I would not buy another one. It's sad to see a nice animal fall to pieces...