Every small or large pond is susceptible to an algae bloom. Growth occurs through the reaction of sunlight on the normal nutrients of the pond, some of which are brought in by natural forces like rainfall, and others that are brought in by the fertilization of cultivated plants. Blocking sunlight, limiting the amount of added nutrients, and possibly using algae-eating animals can keep blooms under control, but once it gets out of hand, there is only one real solution.
Purchase a recommended algaecide available at farm or garden stores near you.
Turn your filtering system off and add the required amount of algaecide to the pond water. The directions for adding any algaecide will be found in the product's directions.You will need to make rough calculations about how many gallons of water are in your pond, then add a relative amount of algaecide to kill off the existing bloom.
Wait for the specified amount of time for the algaecide to work. This will generally be about two days. The algae will turn brown as it dies off.
Dip your pond net into the water and scoop out as much of the dead algae as you can. This will be a wet and messy business, but it needs to be done in order for the pond to come back to normal pH. Also, removal of the dead algae will keep your filter from clogging up.
Turn your filtration system back on and allow the pond to get back to normal. You may have to clean or change your filter medium a time or two to remove any leftover dead algae.