"Iain" wrote in message news:hBslc.81$i24.60@newsfe1-win...
I notice that UV sterilisers are used in ponds to remove 'green water'

would this do the trick here ?

Wouldn't you prefer filter feeder animals benefit from water borne algae?
I know some people who killed some animals introducing UV or more effective
mechanical filters. I killed myself red scallop just by using filter sock.
It was doing fine for months, and died weeks after I added a filter sock.
In my opinion a perfect example of water lacking appropriate food particles.