Phil I have not heard that Koi can solve your watermeal problem before. You have a large pond and even if the Koi did eat the watermeal it would keep multiplying over the areas feasted on. Sorry to say Fluridone is your ownly option until something else comes along. I used to work for the manufacturer of Sonar and would prescribe max rates for watermeal. 90 ppb. Which means if you had a 3.5 acre pond with an average depth of 5 feet you would need 136 ounces of Sonar. Could get pricey and would take out the rest of your vegetation.