Pea-green soup water in ponds is the result of a bloom of single-celled planktonic algae. This algae is free-floating algae, which is different from string, or filamentous, algae. Free-floating algae requires different control methods than those required for string algae. Compared with string algae, planktonic algae is much easier to deal with. Being able to recognize what causes these single-celled algae species is an important part of this equation.
Planktonic algae is caused by an excess of nutrients in the water and not enough “critters” (bacteria, invertebrates, and good algae) to use them up. Most of these critters’ homes are in a layer of life that I call the PATINA of a pond. Most ponderers would recognize this as the “slime layer.” It grows on everything that dissolved oxygen (DO) -rich water touches – rocks, the liner, plant pots, etc. These microscopic animals also float around in the water, but most exist within the patina layer.
The amount of algae (and thus the clarity of the water) can vary, from a just some algae to a huge algal bloom (i.e., pea-green soup). Cold-water temperatures inhibit algal reproduction and growth. This is why most ponds are clearest in winter. Clarity can vary from zero inches of visibility to several feet. Ideal clarity is when one can see the bottom of a 30-inch deep pond. Do not expect swimming-pool clarity.
Any one of the following 30 causes may be enough to create the conditions necessary for a planktonic algal bloom. It is possible that a combination of several of these causes is responsible. The first eight causes discussed below are the most frequent reasons for a planktonic algae bloom, creating pea-green soup water. To achieve the clearest water, our goal is to reduce the excess nutrients and create a balance of the best conditions for the critters. A pond itself serves as a biological filter, so it must be treated as a living, breathing (i.e., needing oxygen) ecosystem. Every one of the following causes relates to this goal.
#1 – Your pond is new, and microscopic animals have not been able to reproduce and populate it yet. You can speed up the process by adding bacteria. However, a pond is not fully mature for at least two years, even when bacteria are added. A mature pond has a very diverse critter population that includes more than just bacteria. It takes time for them to show up.
#2 – The fish are overfed, or they can’t get to the food before it dissolves. These excess nutrients are readily used by planktonic algae. A good example of this is when the food gets sucked into a skimmer box and ground up by the water pump. This is very important: make sure all the food is eaten before it sinks or is sucked in by the water pump. I recommend feeding only enough food so that it is all eaten within five minutes.
#3 – There or too many fish, or the fish are too large for the size of the pond they are in.
#4 – There is not enough water circulation in the pond. Pump a MINIMUM of one pond volume every two hours. It is even better to circulate one pond volume every hour. More circulation means higher dissolved oxygen (DO) levels to all parts of the pond – top to bottom and from one side to the other. Higher DO levels allow larger numbers of critters to exist. This rule of thumb basically applies to ponds smaller than 10,000 to 20,000 gallons. Once you pump 10,000 to 20,000 GPH, you have such high circulation that this rule can be ignored.
#5 – There is buildup of dead organic matter (such as tree leaves) in the pond. This may also be a source of tannins.
#6 – Something died and is decomposing in the pond (i.e., fish, frogs, or toads).
#7 – There are very few aquatic plants in the pond. Certain plants can use up a lot of organic nutrients. Use a variety of plants that take their nutrients directly out of the water, such as submerged plants or plants whose roots dangle in the water. Water lettuce, water hyacinth, and plants that are planted on floating island planters are good examples. These make excellent “veggie filters.”
#8 – The pond has been scrubbed or power-washed, thereby drastically reducing the number of critters. In this case, see #1 – New Pond.
#9 – Aquatic plant fertilizer tabs may not have been pushed completely down into the soil, thereby releasing some of the fertilizer directly into the water.
#10 – Plant pots are leaking soil or fertilizer. Mesh baskets or even pots with holes in them can allow fertilizer to leach into the water. It would take many water lily mesh baskets for this to be a potential problem.
#11 – Water has been added from a lake, stream, sump pump, or drainage ditch. Even shallow well water may add excess nutrients. High levels of nitrates are often present in surface water and shallow wells, especially in areas where the ground has been fertilized.
#12 – Runoff from the surrounding ground has flowed into the pond after a rain. This can bring in all kinds of things that can create green water, including herbicides, fertilizers, soil, and/or any kind of organic matter.
#13 – The pump intake is beneath the waterfall, preventing good circulation. Locate the intake of the water pump at the other end of the pond, away from the waterfall.
#14 – The water pump is shut off (at night, for example) or power to it is interrupted for a period of time. This allows the waterfall/stream to dry out, which kills the patina of life in the stream. This also reduces the circulation in the pond.
#15 – The filamentous algae (string algae) was recently killed by an additive. The decomposing algae itself can create conditions for green water to develop. Even though it can look bad, this algae can serve as an effective veggie filter, so killing it actually removes this type of filter. Remove dead string algae if you can get to it and replace this type of “veggie filter” with one that is desirable.
#16 – Certain pond additives have been used or overused. Fish medications, algicides that contain copper, and other additives can kill the patina’s critters.
#17 – Herbicides or insecticides have been used in or around the pond. See #16 above.
#18 – Lawn fertilizer has been overspread and some of it has gotten into the water garden.
#19 – Night crawlers are dying in the pond. This is difficult to determine in a pond with a rock bottom. This is common in new ponds, where worms used to crawl through the area where the pond is now. They now encounter a rubber liner and crawl up and over the edge into the water. This is more of a temporary situation.
HINT – Smell the pond water. I like to get my face right down to the water’s surface to smell it. If it’s healthy water, it should smell like garden soil: fresh and earthy. If it stinks like rotten eggs, especially when debris is dredged up off the bottom, then we have very low DO levels. The rotten egg smell is from toxic hydrogen sulfide gases, which can allow pea-green soup algae to bloom. These gases are also fish killers if the cause(s) are not addressed. Immediately remove the excess debris and increase circulation.
#20 – Bird feeders are hanging near or over the pond or stream, allowing birdseed and bird droppings to accumulate in the water.
#21 – A large amount of bird, rabbit, dog, or other animal droppings are getting into the water. This can also occur during the winter when there is ice on the pond. The droppings accumulate on the ice, and when it melts in the spring, the droppings go to the bottom.
#22 – Ducks or geese have access to your pond. This is very similar to the above two causes, but some people do not think of the droppings when the cute ducks are swimming in their ponds. Their droppings could be a problem.
#23 – The pump intake is becoming plugged, thereby reducing circulation. This can also shorten the life of your pump. Use a large enough pre-filter to catch the debris before it gets to your pump’s impeller.
#24 – Pump output is restricted, thereby reducing circulation. Inadequately sized outlet tubing can restrict the flow. Always upsize the outlet of your pump by one size (for example, if there is a 1-inch outlet on the pump, use 1 1⁄4-inch outlet tubing). Other restrictions can be caused by a kink in the tubing, organic matter buildup (i.e., algae) in the tubing, or even the filter that the water is pumped to becoming plugged.
#25 – The tops of aquatic plant pots are not covered with large enough gravel. Goldfish and koi like to dig into the dirt – it is their nature – and the soil that is released into the water contains nutrients like phosphate, which algae grows on. It also may contribute to cloudiness from suspended soil sediments. Goldfish can move pea gravel, so use at least 3⁄4-inch river rock. Large koi can really do some serious digging, so go with two-inch river rock on top of your dirt when koi are present.
#26 – A high proportion of “new” water is being added to “old” water. “Old” water is “living water,” containing lots of critters, whereas new water is relatively devoid of life. Using roof water by diverting downspouts into the pond can literally flush a pond of its healthy old water in a heavy rainstorm.
#27 – Lots of rain is falling on the surface of the pond. This may cause an algal bloom due to the nitrogen that is dissolved in it. If this is the cause, the pond’s ratio of critters to nutrients is too low anyway.
#28 – Aquatic plant soils with high organic content, such as compost and/or manure, are present. The excess nutrients can leak out of the pots. Staining of the water by released tannins can also occur, thereby reducing the clarity. As a side note, it has been shown that the compost in the soil can retard the growth of your aquatic plants. The decomposition of the compost uses oxygen, robbing the plant roots of an already limited supply of oxygen. This oxygen in the plant root zone is critical to good plant growth.
#29 – The pond was emptied and allowed to dry out. If this is the case, then a lot of your patina layer died and the critter population will have to rebuild itself. The addition of bacteria will speed up this process.
#30 – The pea-green algae may not be algae at all. It could be a type of protozoan (Euglena) that has chlorophyll (which causes the green color) in its tissues. Euglena are rare in this type of situation, so please do not assume that your green water is caused by these protozoans. A microscopic examination will determine if Euglena are present. Other than their role in less-than-idea water clarity, these types of critters are not harmful in a pond. However, they can be hard to control. A UV Clarifier will work on them. I normally do not recommend UVs, and in this case only would I use one. UVs are expensive and can hide an underlying problem that could stem from one of the other causes listed above.