They are a pretty neat fish. I am partial to the yellow clown gobys myself and I do have a green as well. I had heard that yu can not have nore than one in a tank due to fighting myself. I had ordered 10 yellow clown gobys online from a vendor and they packed each one in a separate bag. BUT my lfs here has quite a few tanks with corals in it, and he has as many as 5 or 6 of them in each tank. He has yellow, citron, green and black varieties and he stated he never observed them squabbling. I have since taken down two PICO tanks and combined the contents and fish (yellow clown) into one larger tank of 30 gal so that makes that cube of 30 gal having 4 yellow clown gobies in it for some time now and not any problems, and they all seem to do just fine together......