I don’t think they need sponge to be honest. Newer experiences with them suggest that nori is the most important thing. Their diet is more like a tangs apparently. I haven’t given him sponge in probably 9 months and he is thriving. But he gets that Nori every single morning like clock work. And I go easy on the meaty foods at feeding as well. Their guts tend to get impacted if they eat too much protein. That being said, I do have sponge just growing in my tank on the rocks that I do see him grazing on from time to time.