There are thousands of types of algae in the world and many of them can be found right in our backyard koi ponds and water gardens. Algae, in general, is mother nature's filtration. It tends to bloom when the koi pond is out of balance for whatever reason. Sometimes this is temporary like in new koi ponds or in the Spring. In the Spring, the filter usually takes awhile to get running after its Winter dormancy, and you will be temporarily under filtered during this process. So algae blooms in the Spring are very common. Other times it is a permanent problem because of water quality issues in the koi pond. Usually poor water quality will come from being overstocked or under filtered, or both. Also, rocks on the bottom of the koi pond can trap waste and create a feast of nutrients for algae. Now, water quality issues do not always mean poor filtration, it is also possible your source water is high in phosphates or nitrates which are providing food for the algae. These should be tested before you take any dramatic steps to remove algae.
Some of the more common types of algae found in a koi pond are green water or single cell algae, string or hair algae and blanket weed. There are different varieties of each of these types found in different regions, but in general, the treatments are the same. You will also find a nice green carpet of algae growing on all underwater surfaces. This can be anywhere from 1/4 inch to 2 inches long. This is one to the best things for the koi pond and should never be removed. Luckily most algae treatments won't effect this type of algae.
Treatments for algae come in all shapes and sizes, natural and chemical, expensive and cheap. Choosing the right treatment for your particular problem can sometimes be an expensive trial and error process. Hopefully I can cover some of the more general problems and remedies that effect most koi ponds and help you come to the right solution.
Single Cell Algae is one of the most common forms of algae affecting koi ponds. It turns the water into pea soup and you cannot see your koi for weeks or even months. This can be quite frustrating for the koi hobbyist. One thing to keep in mind, usually the green water is extremely healthy and the koi and very happy in this environment. The alga is an excellent natural filter and many times when the green water clears, you are surprised by how beautiful the koi look. But that doesn't mean we want green water, after all, you want to see your koi, right? So let's look at a few of the causes of green water and then we can talk about a few of the solutions.
Many times the green water algae are found in brand new koi ponds. This is to be expected and generally, it's best to just wait it out. The water is turning green because the filter has not established itself yet. The algae are Mother Natures way of keeping the water healthy during this process. If dramatic steps are taken during this time to remove the green water, it can be unhealthy for the koi. Sometimes it can take several weeks if not months for the new koi pond to clear up. So be patient, if your pond is properly designed and has the correct number of koi, the water should clear all on its own.
If over time the koi pond does not clear or does not develop the type of clarity you would like, I highly recommend a UV filter. A properly sized UV filter can clear up a koi pond in just a few days. As water passes through the UV, all single cell algae are destroyed. Sometimes, if the UV unit is smaller, or the flow is a bit too fast for the size of the UV, the algae will merely become mutated, and unable to reproduce. This is usually adequate to clear the water. The UV filter only works on single cell type of algae in koi ponds. Other types of algae won't ever pass through the UV unit and will not be affected by the UV light.
Some koi hobbyists have had luck using barley straw to remove single cell algae. This has historically been hit and miss. Some people swear by it, others swear at it. The theory behind why barley works in koi ponds is in the way algae dies. When an algae cell dies, the membrane bursts and released from the cell is a chemical that will actually kill surrounding algae cells. Now, this often doesn't matter as it will reproduce much faster then it will die. The theory with barley is that when it decomposes, its cells release a similar chemical to that of the dying algae cell and the algae start to die off. It has been said that the decomposing barley releases hydrogen peroxide, and that is why the algae die off, but if that is the case, why don't we just add hydrogen peroxide to the pond and save the mess of barley?
String algae can also be a real nuisance to the koi hobbyist. It takes the form of a long string and hair algae that can very quickly take over the koi pond. It attaches itself to the walls and rocks and waterfalls and grows very quickly. Like green water algae, string algae does not cause health issues. If is another type of natural filtration and often provides excellent water. The koi will also eat this type of algae. Because of this, some koi ponds never have trouble with string algae. And one of the more natural ways to get rid of string algae is to stop feeding the koi and let them eat the algae instead. It's not highly nutritious, so don't make the koi live off string algae for more than a week or so without returning to a regular feeding routine.
If the koi are not able to eat it fast enough and it is starting to cause mechanical problems, like clogging drains and skimmer, then something needs to be done. String algae can cause significant damage to pumps and filters and if you are out of town, you may come home to a damaged pump and stressed koi. Physical removal is the easiest and cheapest way to keep string algae under control in a koi pond. However, this can be difficult, especially in large and deep koi ponds. The first thing to look at is the filtration. The algae are growing for a reason, usually because there are excess nutrients in the pond feeding the algae. Adding filtration and doing more frequent water changes will often correct the problem.
Some koi hobbyists get frustrated and start dumping in algaecides to kill off the string algae. If you decide to go this route please keep two things in mind. First, never use an algaecide as a preventative. Wait until there are actual algae to treat before adding any algaecide to a pond with koi in it. The reason is, if there are no algae, the algaecide will remain in the pond until it reaches with something, namely the koi. So I think the age old saying holds true here....if it ain't broke, don't fix it. Second always use a little less than the labeled directions, rather then a little more. One other thing to keep in mind is most algae products will dramatically reduce the amount of available oxygen in the koi pond. You must make sure all the pumps are running and it may even be necessary to add additional aeration during the treatments.
Another more natural way to reduce string algae in a koi pond is by using koi clay. Koi clay is a mineral rich clay that doesn't directly kill the algae. In fact, in all my research I have not been able to find an exact reason why koi clay works so well on string algae. But it really does. I have been told that when using an abundance of koi clay it can smother the string algae. Others have written that the clay, being so absorbant, removes toxins and nutrients from the water causing the algae to die off. My theory is that koi clay makes the water more healthy. It aids is the koi's digestion, it helps the filter, and in general improves the water quality in the koi pond. Better digestion in the koi means less waste and better filtration means fewer nutrients for the algae, so it stands to reason the algae will die off all on its own. I have personally seen time and time again how koi clay prevents string algae. The most effective way to do this is to physically remove as much string algae as possible from the koi pond, then dump in 10x or even 20x the recommended amount. There is no danger in doing this, it's just clay, millions of year old dirt, nothing added, nothing is taken away. It will cloud the water, maybe even for a few days, depending on how much you add. But keep adding it and after a few weeks, you should see significantly less string algae in your koi pond.
So, I hope I have helped you discover some of the causes behind your algae and some of the best methods for removing it. Remember, algae are natural and healthy and the koi love it. Lakes and rivers are filled with it. It's us silly humans that demand perfectly clear water in our koi ponds and equate this with healthy water. But next time you want to get rid of your algae, think about the last time you went to a lake. How deep were you able to see into the water?