They do best with low amounts of lighting, and very hard currents. Otherwise, you will find yourself constantly blowing algae off the sponge. Algae will clog the pours of the sponge, causing it to soffocate and slowely die off. Air will also get caught in the pours, cuasing it to soffocate. I would recommend placing it in a dark area of the tank, or under a ledge, to prevent any algal growth. Hard currents are also needed on the sponge, as NM Reef already stated. I personally think that sponges are not the easy invertibrate to care for. I've never seen someone keep one healthy and thriving for over a year.
Graham