My advice is remove it immediately. I just researched several places in regards to my own shrinking tree. They can release nasty toxins in the water when they are dying. Sponges do not have much chance in the aquarium life span is usually 8 months at best. We gotta find something else bright colored, with a tree shape for these guys. I hate the thought of losing my stock by trying to make them happy. :sick: I removed mine today and suggest you do the same. I also bumped up the carbon in the system.
Good Luck Cindy