Pond advic

So I have inherited a 25x25m enclosure with a 3mx2m oval shaped pond, about 1.5m deep. It is a liner with no inlet or outlet, just a whole in the ground full of water. Realistically is there any way of keeping this clean? I can do 50% water changes weekly if I need to with a petrol water pump I have. What would my options be? I have kept tree dicks for a while but always used tip and fill containers for water. I would like to try and squeeze a few pairs of wildfowl, laysan teal, white faced duck, argentine red shoveler, if possible a pair of red breast and nene.