A biological filter takes fishy waste and turns into plant fertilizer. Algae
loves this stuff. A biological filter is good for fish, not to hot to get rid
of green water.
That is why we recommend adding a veggie filter or plant filter.
Basically this is a container through which the pond water circulates and is
filled with plants.
The plants take up a lot of the nutrients and the idea is to deprive the free
floating algae of lion's share of the nutrients that it was getting. The plant
roots will also catch the mulm and crud which can foul the pond.

A plant filter can be as simple as floating water hyacinth in a stock tank or
growing watercress in a rock waterfall (this is what I use) to the even better
filters that Ingrid, BV and Bonnie use. They will probably stop in and post
their sites to give you an idea of the contruction.

In addtion to the plant/veggie filter information I'll post the algae tips in
an additional post in case you are suffering addtional problems that lead to
green water.

kathy :-)
(still use ka30p for email
this acct. is for reading rec.ponds only)