My onyx are about 13 month to be exact,I have 3 tanks 34g 20g 12g my 34g I have 4 they don't fight they get along fine I have ducans ,frogspawns,hammer ,n a green leather coral,witch they host and all that good stuff clownfish do my other tanks only got rocks ,the one on my 34g all 4 are almost all black,the ones on my other tanks are only showing traces of the black faded ,so I'm thinking is that they feel better when they host something and not stress out its in there nature to host something some people say ,its the age yes I agree,other say the lightin others the diet, I feed them all the same