I have been thinking about getting an in-line CO2 diffuser, as it annoys me having to have one inside the tank (as my tank is very small). Does anyone have one that they are using on a smallish tank (mine's 54 liters) are they any good? Any problems?? Also is it just Cal-Aqua Labs that make them? They are quite expensive Thanks EDIT: Just found this http://uk.youtube.com/watch?v=yuh0gUDYpuo looks good. I like those rocks too!