I have a 55g SW that has been set up for over 2 yrs with only 1 fish- a C. solandri puffer, full grown- in the tank with about 3-4 blue claw mexican crabs and few margarita snails (they don't last long- the puffer loves them). The tanks has a few corals- 1 frogspawn, 1 toadstool, 1 kenya tree, 2 finger soft leathers, some star polyps, and few mushrooms. All the corals are doing well. I had purchased about 1 year ago a small discount frag of sps that had a pretty purple piece of sponge on it. The frag didn't make it but the sponge did (despite being exposed to air while bagging). The original piece has been exposed to air a few times (water changes, live rock bagged for moving this summer) but has never died. Instead I now have sponge on every piece of live rock- not a lot but some. And it is not just purple- have yellow, green and white too. Is it good to have this encrusting sponge or will it kill off the encrusting coral that is starting to really grow on the live rock?