These little blue fish, called neon gobies, have set up a "cleaning station" in this Reef Ball(tm). Other larger fish (the Snook and Atlantic Spadefish in the background) let the neon gobies clean parasites off of their scales and around their mouths. Their bright colors alert the larger fish of their services so they don't get eaten. Neon gobies are the smallest adult marine fish at just a little over an inch when full grown, however they lay some of the largest eggs for marine fish and can actually be bred in an aquarium
