If natural remedies for your algae problem didn’t work to your satisfaction, there are chemical remedies that can be exercised with caution.
Remove as much algae as possible before starting chemical treatment. Maximum effectiveness of ridding algae occurs at 50 degrees Fahrenheit.
Just as sunlight will feed plants as well as algae, algicides will slow the growth of algae as well as other plants and high doses may kill them. A way of avoiding too high dosages is to perform a fifty percent water change and then treat the water with a half dose.
The algaecide Potassium Permanganate is only to be used on cool, cloudy days when the water isn’t too warm. If used on sunny days, the treatment will turn the water a murky yellow. A maximum of three applications on three successive days is usually recommended. At the end of the treatment remember to remove the dead algae.
Formaldehyde (37 percent solution) may be used in the rate of one drop per gallon of pond water to kill floating algae. A possible killer of lily cultivators, it is wise to remove plants before treatment. Fish will not need to be moved as the 37 percent solution is safe for killing fish parasites but will not kill the fish. The person handling the solution should avoid skin contact and breathing in the chemicals.
For larger bodies of water, Copper sulfate can be used. Dosages vary and will usually be used on alternate days over a two week period if there are fish in the pond. Use Copper sulfate with great caution because a too high dosage can kill your fish by way of asphyxiation.
Algae can certainly be an annoying problem to deal with while maintaining a water garden. Fortunately, there are many ways to keep an algae-free pond but a good place to start is with plants.
Under water plants such as Elodea, Hornwort, Sagittaria and Cabomba are some primary plants to consider in this equation. Ideally, one bunch for each square foot of the pond’s surface is enough. If you have a high population of fish in your pond, consider more plants to counter-balance fish waste and their grazing habits. Elodea is a great plant to help control algae because it breaks dormancy earlier than other submerged plants. Usually in bloom a month earlier than others, you’ll get a head start on controlling algae. Keep in mind that Elodea grows and spreads rapidly and will need thinning by summer.
If your pond water is already green with algae, it will take longer (usually up to six weeks) to get the problem under control. Under these circumstances, submerged plants should be moved closer to the water’s surface where they can receive more sunlight for faster growth. As the problem lessens, the plants can be moved back to their original settings. Ponds that have up to four inches of pea gravel at the bottom are ideal for the “tame” Dwarf Sagittaria that roots directly into the gravel. Ponds with Dwarf Sagittaria are unlikely to experience any green water algae problems at all.
Sunlight not only helps submerged plants grow but also greatly aids in feeding algae so covering your pond with floating plants is ideal. A general rule of thumb is to cover one-third to two-thirds of the water’s surface with plants such as lily pads and floating plants such as Water Hyacinth, Water Lettuce, Fairy Moss and Duckweed to name a few.
Surface shading isn’t necessary in keeping a pond clear of green water algae, however it is much more difficult to maintain a pond without it and will require many more submerged plants. As stated earlier, Dwarf Sagittaria carpeted along the bottom will help compete with algae for the available nutrients.