not sure, but i have a 20 g. hex tank and 1 have 3 fish in it. 1 tomato clown and 2 blue reef chromis. 1 is about 3.5 inches and the other is not even an inch. all these fish get along great. the 2 blue chromis dont even swim by each other..lol. they are awesome looking fish. the small one is very blue, and the larger one is lighter colored on the botton but a bluish green on top. when the light hits him certain ways he looks purple. very cool looking. the green chromis dont have alot of color at all. they are very dull in color. i read that the green chromis are open water fish and the blues like to stay closer to the reef. i have a fish only tank and they seem to be doing just fine. not sure how the blues and the greens are together, but i would go with the blues for sure. the blues are a little more expensive though. i paid $20 for the big 1 and $15 for the small 1.