The porcelain crab is a filter feeder and should get what it needs from your water. Maybe some liquid invert food if you want. I don't have alot of experience with sponges. My sponged died.
I know you have to keep them in good flow so algae and things don't settle and grow on them. Also, you have to be sure they never come out of the water. When adding them to the tank, you'll need to submerge the bag for removal so the lfs
water will be added to your tank. Be sure you are buying from a respectable, trusted lfs