I would recommend using those nylon pot scrubbers. I bought a lot of
them at Big Lots for 99 cents for a bag of 9 I think. I would say that
a five gallon bucket is probably big enough. Do you have plants in
your pond? I have a lot of plants (water hyacinth, water lettuce,
irises, horsetail, etc) in a pond that is a little smaller than yours.
I think the plants do most of the work. My filter is a mesh plant
basket containing scrubbers, and my pump. I cut up a plastic coke
bottle to maintain a clear path to the pump inlet. I topped the basket
with plastic gardening fabric.