I actually have that red one from liveaquaria see pic below.
You can see the red to the right and again the purple to the left.
That purple to the left is actually the exact same sponge in the previous post.
The previous post is what it grew into but I had moved the red sponge away....
You can all see a yellow ball sponge that I have had for many years in the picture...
Honestly they do not seem to need much at all..
I add silicate to the water for my sponges but I am not sure photosynthetic sponges actually use them for structure.
I also add phyto, bacteria, and other fine particulate food like reef roids. I feed these mainly for other sponges and gorgonia in the same vat.
I can not be sure the photosynthetic sponges even use these. I find them much easier than any other sponges and very fast growers..