Things You Will Need
One of the least invasive ways to treat a fish pond without removing fish and draining the water, is to treat with a 3 percent solution of hydrogen peroxide. This is the common concentration of hydrogen peroxide sold in grocery store and pharmacies.
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Measure the width, length and depth of the pond with a tape measure to calculate approximate water volume. For example, a fish pond might be four feet wide, six feet long and two feet deep.
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Multiply the three measurements to calculate the cubic feet of water in your pond. For example, 4 x 6 x 2 = 48 cubic feet of water. There are 7.48 gallons in one cubic foot of water, so multiply 7.48 by the total cubic feet in your pond. In this example, 7.48 x 48 = 359.04 gallons of water in the pond.
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Use one pint of 3 percent hydrogen peroxide for every 1,000 gallons of water. To measure the amount of hydrogen peroxide to use in the current example, divide 359.04 by 1,000 for a result of 0.36 pint. This is slightly more than one-third of a pint. One pint contains 16 fluid ounces, so 0.36 pint equals 5.76 ounces (16 x 0.36 = 5.76).
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Pour hydrogen peroxide into your measuring cup based on the calculation for your pond size, then pour the solution into the pond. Stir gently with a wooden stick or metal rod to circulate the solution.
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Remove the dark slime of dead algae from the pond surface with a skimmer the day after treating with hydrogen peroxide.