Ok, I have not had the best of luck with these little Diamond Goby kritters. My favorite fish though. I have noticed that some have black under there mouths. What is this? Is it bad? I looked all over for one because my sand had the brown glaze of diatoms and needed cleaned....I had to settle for this pair that had the black under their mouths. I just got them today so we will see. But, I sure would like to know what that means. Thanks.