The bubble bead filters use gravity to drain the water sucking air in an inlet at the narrow waist line area of the filter. My experience is that it does not provide enough force to backwash out the algae that does find its way in. There is an improved plumbing arrangement where the pump can feed through the outlet at the top during backwash which makes it a better filter, but the bead filters that use the air blowers for the cleaning are much superior. I have a BBF-2, which is rather old, a SacKoi 15, and a Aquadyne 4.4B, and truely prefer the SacKoi filter.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard