Answer: Algae is actually a sign of a healthy pond, but it can get invasive and most people consider it unsightly. Pond owners can purchase beneficial bacteria in either a dry or liquid form and pour it into the pond. The bacteria requires the same nutrients as the algae, so it basically "starves" the algae by consuming its food supply.
Chlorinated water doesn't contain this bacteria and although it will occur naturally over time, the process is often too slow in an artificial environment.
A biological filter, sometimes called a biological clarifier, simply provides a location where bacteria can easily colonize. The filter keeps algae away and has moving water, which provides an oxygen source for the bacteria. Used together, the filter and the beneficial bacteria will help keep your pond's water clean and clear. However, the bacteria go dormant from late fall to early spring when temperatures cool. Add the bacteria every two weeks in winter. Bacteria is dormant when you add it and it takes time to come out of dormancy. Be patient because it won't work overnight. If the algae's appearance bothers you, try scuffing up a rod, such as a broom handle, with sandpaper and then using it to "twirl" up the algae like spaghetti.
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