The pink/ orange ones that have hair like tentacles move around alot. They pack alot of punch and killed a waratah anemone and pizza carpet. The ones that tentacles stick straight out and have vibrant (usually green) centers stay put very well. They don't need much if any light and eat like a haus. Def. worth buying the second type. Oh yeah substrate, they make their tube with their own mucus like feather dusters. I had crushed coral and stuck pvc in the substrate for him.