Opcn
Big fish
I wouldnt suggest topping up the pond, the chlorine may be diluted but it only takes a little bit to kill a cycle and thats a massive fish die out. What you mat want to do is get an air compressor with a long hose and somesort of diffuser at the end, Then plug it in and toss the diffuser into the middle, kick that DO level right up. William has the right Idea with digging out the silt, it would be great in gardens as an amendment to the soil.