Unicellular algae is the type of algae that pond/aquarium owners have the most problems with, it is the cause of green water and multiplies quickly when the conditions are right, water temperatures above 15°C, excess nutrients such as nitrates and phosphates combined with lots of sunlight and before you know it your pond is so green that you can’t see the fish, this algae is not harmful to fish but it surly spoils the pleasure of watching them.
Keep in mind that that the problem is caused by excess nutrients so the cure is to remove these nutrients not kill the algae. Due to it’s small size it is difficult to filter this algae from the water, paper cartridge, diatomaceous earth and sand filter will filter the algae from the water but block very quickly.
The best way to control unicellular algae is to use products that remove phosphates from the water this will effectively starve the algae, as with all methods where you kill or starve the algae the dead algae cells have to be removed from the pond or they will decompose and release nutrients back into the water.
If there is rain run-off into your pond it will be almost impossible to control algae, water flowing into your pond from lawns and plant beds can carry with it nutrients in the form of organic or commercial fertilizers, if this is occurring take measures to divert rain run-off away from the pond. If the rain run-off flows over concrete and then into the pond it will raise the ph of the pond water, which will contribute to algae growth, a ph closer to neutral will reduce algae growth. Any water flowing across cement, limestone or marble will increase the ph of the water.
A relatively recent method for algae control is barley straw, the use of straw to control algae has been known for hundreds of years. 400gr of straw placed in moving water will control algae for about a year in a 4000 liter pond, it will take 6 to 8 weeks to start working, but it will tan the water. As the straw slowly decomposes it releases natural algaecide and provides a source of food for you fish, this method will not kill algae but it will stop new algae from forming. It will not reduce the cause, which is excessive nutrients, the most natural methods to control algae is to properly stock the pond with the correct amount of fish in relation to the amount of plants and the size of your pond keeping the number of fish to 1” of fish for each square foot of surface area should not overload the waste control of a balanced pond.
A pond should have enough plants to shade 60% to 70% of the pond surface. Plants should consist of both bog and marginal plants as well as submerged grasses and lily-like plants.
The leaves will shade the pond and stop excessive sunlight reaching the algae, while the plants remove the nutrients from the water and provide refuge for fry and small fish.
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