I would get all the shells out if i was you... I have heard that you can have higher nitrates and high amonia levels due to too many shells as they trap stagnant water... Thats just what i have heard, I take all my empty shells out. For a 55g I keep about 10 hermits total (7 blue leg, 3 scarlet) and like 5-7 turbo, and about 15 nasarius, and an Urchin and a yellow tang. I dont see any bad algae at all anymore..... The urchin is one from down at Port Aransas, and he seems to clean everything up very well, doesnt even harm anything... But he may possibly be the one eating the good purple algea though, which doesnt matter much though.