you need ag lime. don't buy it from the feed store...call your local dirt/gravel supply company...7 dollars per ton or say 70-80 bucks a dump truck load depending on the size of dump truck. i think ric s is correct..and i think the number may range from 2-4 tons per acre. so you would need about 6-12 tons. or about 1 dump truck (usually a 10 ton load) which would run you as stated in my neck of the woods 70-80 bucks. have them dump it in your water in flow area, then throw as much as you can out in the pond, leave the rest at the inflow...when you get rain it will wash it in...don't kill yourself spreading it nicely...the water will carry it in...at least it did for me. i threw out about 20 tons...then said screw it...and left the rest in the water inflow area. rained hard...washed the lime in nicely. i think there are some good chapters in the "Basic Pond Management" book offered from the home page...that's where i figured out how much to put in...and it did me right. one more thing, ag lime kinda feels like sand, and stays put pretty well from what i noticed. sooo, i wanted an area i could walk into the pond without getting all muddy, so put the ag lime about 6 inches thick about a 10 yard by 10 yard area...makes a nice "beach". call me crazy, but seems to feel nicer between the toes than the mud or clay!!!! plus, with time and wind...the wave and wind action trickles in the lime.