Check the Water
Ponds can be muddied by livestock, carp, bullheads or crayfish that feed on the bottom. Ponds muddied by heavy rains or flooding should settle out in about a week. To check the degree and persistence of the muddiness, collect a jar of water from the pond. Suspended clay silt that doesn't settle to the bottom of the jar within a week indicates a problem.
Amounts and Application
The amount of recommended alum in treating pond water varies slightly. The Kansas Department of Wildlife and Parks says that alum is acidic. If the pond water is acidic (low pH) or is soft, first add about 20 pounds of hydrated lime (calcium hydroxide) to each acre-foot of water---the volume of water, 43,560 cubic feet, that will cover 1 acre to a depth of 1 foot. To determinate acre-feet, multiply the width of the pond in feet times the length times the average depth. Dissolve the alum in water. Using small boats, quickly spray about 50 pounds of alum for each acre-foot of water. Do this on a calm day. Wind will cause waves to break up the mass of alum, preventing it from settling.
The Ohio State Department of Natural Recources says if the level of pH drops below 7, then add 50 pounds of hydrated lime for each surface acre. Ohio recommends 25 to 50 pounds of alum per acre-foot. An initial treatment 25 pounds per acre-foot should clear the water in a few hours.
If there is no change after one day, add another 25 pounds per foot acre. The Ohio resource managers say the best way to do this is with a sprayer in a small boat. In large ponds, pour the alum into the prop wash of the boat's outboard motor.
The Mississippi Department of Wildlife Fisheries and Parks tests four 1-gallon containers of pond water, adding 1 tbsp. of alum mixed with water to the first, 2 tbsp. to the second, 3 to the third and 4 to the fourth. The containers are checked 12 hours later. The container with clear water at using the smallest amount of alum determines the application for a particular pond. For each tbsp. used, Mississippi adds 30 pounds of alum and 13 pounds of hydrated lime per acre-foot.
The Virginia Cooperative Extension service recommends 150 to 300 pounds of alum per acre. Low acidity rates of soft water should be countered by a ratio of one part alum to half part limestone. The chemicals should be sprayed or poured into the prop wash.