Thanks for the info.
I have got a 20kg bag of gypsum earlier today which is 85% gypsum and 5% calcium hydroxide (the lime you talk about). I think I may add this tomorrow to the pond and see what happens. If there is no improovement then I'll pick up some alum on monday. I feel it can't hurt any to add the gypsum. Do you think its adviseable to turn off the areation while adding gypsum also like alum? I just wonder if its good to add the gypsum with areation on for 30 minutes at full blast and then turn off given I have a smaller pond.
I have found a source for the alum but need to find a source for the hydrated lime.
