I have 6 ducks in a large soft fruit enclosure, about 15 x 9 metres, (yes the polypipe construction worked beautifully) with an insulated shed for sleeping in. We installed a triangular spa, purchased cheaply from the tip, and this is raised a little so I can drain out the sludge once a week. I take out 70 to 90 litres but much of the smelly foecal material floats and there are about 400-500 litres of water in the spa. I can't waste that by draining the whole thing each week. Any ideas on safe flocculants or anything???