I put 7 in my 80 gallon tank, 2 per rock structure and one for good measure. The males slowly fought each other over about 6 months for territories, now I think I'm down to 1 big male and 3 small females. In my experience they will forage for any other algae before they try to eat whole green bubble algae, I still have a bunch in my tank and even when I had 7 emeralds they didn't put much of a dent in the bubble algae that was growing. I did find that the crabs and tangs will both readily eat the bubble algae, if it is popped/dying. I go in every couple of weeks now and scrape a bunch off the rocks and scoop it out of the tank, the emeralds and my tang then eat the little broken bits off the rocks that got left behind and it looks clean for another month or two; doing this I now have way less algae, just a few tufts here or there that I can't get to without doing a bunch of accidental fragging. In short, my advise is less emeralds and rely on them less, go in and remove/pop what you can yourself whenever you can and it will be much easier to manage.