I have a sort of crust which has formed on the lining of my pond. Also, accidentally, I over dosed my fish on anti fungus & bacteria treatment which has caused green matter to grow on this crust which is making the water very dirty, apart from the pond looking unsightly. I have tried to replace half of the water, (the pond is about 400 gallons) but can anyone advise how I can get rid of this crust which has formed? Hosing the sides does help to remove some of it. Many thanks for any help.