I would try Gypsum. Here Gypsum is about $160 per ton and that should be more than enough to clear up a pond that size. Gypsum doesn't work as well as Alum, but Alum here is $50 per 50lb bag. Plus with Gypsum, you don't have to monitor PH levels. To apply gypsum, I just throw a handful at a time into the prop wash of my electric 5 hp motor, it dissolves instantly.