Despite the fact that it is a natural process, the growth of algae in the ornamental pond is something most pond keepers consider unacceptable. The majority of the algae fall into one of two types, free floating planktonic algae and filamentous algae. The only things either of these require to grow is sunlight, water and nutrition in the form of the fish wastes, nitrate, and phosphate. We can try to control the algae by providing competition in the form of ornamental plants for the available food source. These plants can be lilies, lotus, “bog” plants, floating Hyacinths and submerged “oxygenation” plants like Anacharis or Hornwort. The use of shade cloth or an ornamental trellis can reduce the direct sunlight.
Even with the best of “control”, algae are going to grow. The easiest to eliminate is the free floating unicellular algae that causes the pea green water look. The use of a proper size UV clarifier and water pump can kill this algae. Ideally, you need a UV system rated to give an effective “kill dose” at a flow rate equal to or greater than one-half the volume of the pond. There is a catch-22 to killing the free floating algae. Unless you have really good water circulation and a good filter system, the dead algae will sink to the bottom of the pond, and as it rots, provide a great food source for new algae! This is why a lot of the liquid treatments are effective for days/weeks, but the algae does come back and you need to treat the pond again.
Filamentous or string algae is usually attached to the sides/bottom of the pond or on the rocks of a waterfall or stream return. You can slow down the growth of this type of algae with barley straw products. As the barley bale slowly rots in the water, it will promote the growth of a fungus on the bale that releases chemicals that inhibit the new growth of filamentous algae. Please note that it will not necessarily kill established string algae, but inhibits the growth of new algae. It is best to start using the barley products early in the season. Most bales need replaced after 2-3 months. Most algaecide treatments are safe for the fish and plants, or else give you a warning to not use under certain circumstances. Pond Block should not be used with lilies or hyacinths, and most treatments should not be used in ponds with fish grown for consumption. These treatments are also safe for any birds or small animals that might use the pond for drinking water.
About the only danger algae in the pond can pose to the fish or other animals is if the pond develops an algae bloom that literally causes the pond to look like someone poured paint into the water. The danger to the fish would occur at night when the algae would consume most of the oxygen, resulting in a fish-kill. On rare occasions, more likely in retention ponds than ornamental ponds, there may be a cyanobacteria algae bloom that will literally release toxins into the water, poisoning the fish and possibly causing illness to animals that drink from the pond. This is similar to the “red tide” that occurs in the oceans. Again, this is much more likely to occur in retention ponds or ponds with fertilizer run-off from rain, not your average home water garden.