I just bought a home in Toronto and with my home came a small pond in my back yard. I think it is about 180 gallons and it had six koi in it. The guy I bought the house from told me the fish were two yrs. old and he said the pond was maintenance free. After a few weeks i decided to look at the filter and it looked like the guy hadn't ever cleaned or changed the filter. I decided the pond needed some maintenance so i changed the filter,changed half the water and everything seemed okay. Another month later i was at a garden center that specialized in koi ponds and they sold me a product called D-solve by a company called crystal clear. I am pretty sure I put too much in and slowly all my koi are dying. I changed half of the water waiting a week and i am going to change half of it again if there are any fish left. My question is if they all dye what should i do to start over. Also what are the things I should do to ensure healthy happy fish in clean water. I have a pump and a filter in the pond with a waterfall.