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Does Having Too Many Koi Cause String Algae? – Water Garden & Feature Q & A

Picture of String Algae.


Water Gardens & Features Q & A

Q: I have a bad case of string algae in my water garden and someone told me it was because I had too many fish. I have an approximately 1,000 gallon water garden and around 40 koi 6″ long in it. Is that really too many? If so, how many should I have? – Marco of Texas

A: You can ask anyone here at The Pond Guy; usually the first question we ask when someone says they have a bad algae problem is, “How many fish do you have?”, followed by, “What size water garden do you have?”. 9 out 10 times, there are way too many fish in the water garden. So why does having that many fish cause algae? Let me explain.

Sunlight + Food Source = Algae: Algae really only needs two things to grow, sunlight and a food source. The food source can come from many sources but fish waste is a major contributor. This means the more fish you have, the more waste, the more algae. Make sense?

Finding a Balance: When you put fish into your water garden always consider the future. Small fish become big fish and fish are very romantic creatures. Let’s say you purchased 20 koi at around 3″ each. Your pond may be able
to handle the fish load for a few seasons then all things start to change. As time went by, your fish have grown and maybe even started a family. As nature takes its course, your pond starts to pay the price and water quality becomes an issue. Take into consideration that 40 1″ fish produce the waste of just one 12″ fish. So even though you pond may have been able to handle the fish load in the past, you must consider that your fish load or fish waste grows expotentially every year.

Koi vs. Goldfish: I know that when you’re shopping for fish koi are more expensive and sometimes the goldfish look really nice too. I feel the same way myself at times. Just keep in mind that goldfish can reproduce up to 6 times a year where koi only reproduce once a year.

Fish Tips: Goldfish can grow up to 18” long and live up to 20 years, where as a koi can grow up to 36” long and live over 200 years! One famous scarlet koi, named “Hanako” (c. 1751 – July 7, 1977) was owned by several individuals, the last of which was Dr. Komei Koshihara. Hanako was reportedly 226 years old upon her death. Her age was determined by removing one of her scales and  examining it extensively in 1966. She is (to date) the longest-lived koi fish ever recorded (wikipedia).

POND TALK: How many fish do you have in your water garden?

16 Responses

  1. Update: the aerator is causing MORE algae to grow. All four bubblers have tons of string algae growing up from where they are located in my pond. I thought more aeration was supposed to reduce algae.

  2. My pond is clearing up already! The snails have surprisingly cleaned up the bottom of the pond quite a bit. I’m pretty sure the fish have cleaned up alot of the algae around the top because I don’t see any snails up there. I also put an aerator in and that also seems to be helping. I know I probably have too many fish but I am planning on either adding on to my pond or building another one in my yard. Most the fish I have are pretty small and I know they will keep growing but for now they seem very happy and have enough room to move around.

  3. I have a 1500 gallon pond with 2 streams and a waterfall. I have 10 goldfish (2-3in) and 13 koi (10 3-4in and 3 7-9in). I pull tons of string algae out of my pond every 2-3 days or so. My pond is also in full sunlight all day long. I actually just bought 25 trapdoor snails to try to help with the algae. I know they don’t prefer string algae but I am hoping that they will help stop it before grow out of control again. I will keep you posted over the next couple of weeks.

    • Hi Bill – thanks for your reply. You would need to have 250 square feet of surface to have that many fish. To combat the sunlight, try adding some shade with Pond Dye and also floating/marginal plants. Our DefensePAC is also full of natural bacterias that will help maintain your filter and breakdown the excess fish waste in the pond. Be sure to keep in touch and let us know how these products work for you if you try them. Thanks!

  4. we have a pondless waterfall/no fish. We have string algae as well. We manage to take care of it.

  5. I am curious as to, your geographic location?
    Do you keep an eye on water temperature? if so what is your water temp right now (as a daily average)?
    One thing about small volume ponds is they have a tendency to get very warm if in direct sunlight even for several hours. One of the benefits of larger deeper ponds is they tend to stay more temerature stable with increases and decreases occuring at much slower rates.

  6. I too am having a problem with string algae. I have a 550-600 gal pond with about 125 sq ft of surface area, plus a waterfall with a 5 ft stream down into the pond. My fish load is a total of only 7 fish, with an overall summed lengths of about 40 inches, which I don’t believe is excessive. The pond gets maybe about 2-3 hours of direct sunlight during the day (rest of the time it’s shaded).
    So what could I be doing wrong to get so much algae?

  7. I am saddened that your question did not actually get answered by the Pond Guy…
    There is an actual formula that takes into account the surface area, volume and the over all combined length of the fish you have in your pond. You will have to do a search for it because I don’t have it’s location in front of me at this time.
    It is my educated guess that you do have far to many fish for a pond of only 1000 gallons with out knowing the actual surface area.
    I have 65 to 70 Koi in my pond the oldest 9 years old and the majority only 1 1/2 years old. Even though my pond/water feature has a volume of between 15 and 16,000 gallons I am going to have to make the awful choice of giving away quite a few of my loved Koi this season.
    My water feature has 2 – 300lb. Sand filters, a 200 watt UV system, bio-filter and a full airation system. Our main water fall passes 4,200 gph. 3 small water falls of approx. 600gph each and several spitters passing about 120 gph each. I mention these figures to prove we have not cut corners but I know that to many fish with even the slightest balance upset can lead to disasterous water conditions in a slap of a tail.
    Pick the fish you have named or the ones you just love and find a good home for some of the others.
    You and your fish will be glad you did.
    Good luck,
    Mike Loftus

  8. I have 25 fish, most I am pretty sure are gold fish. I got the pond responsibility when I moved, the pond and fish came with the house. The pond was so green when I got here I thought we only had about 6 fish, cause you could only see them when they came up to the top. When I cleaned it out I found 25 of them, some have long fancy fins, some look like normal gold fish and the range in color from orange, white and a mix of the two. They range in size between 3-8 inches.The pond is about 10 feet long X 4 feet wide X 4 feet deep. Pluse we have 2 large and 1 small frogs that have taken up residence too. I am sure it is way to many fish. I am cleaning the filters once a week to keep up with the algae, adding blue tint, and adding a chemical aid to help control the algea.

    • Meghan,

      Maybe start out with a smaller fish load and slowly work your way back up. This is usually the best way to see how much your water garden can handle.

      Thanks for the comment!

  9. In my 1st year of having a pond, I started with 4 Comets-I didn’t want to spend the extra money on Koi until I had more experience with a pond. The next summer I had the 4 original Comets and many, many babies, which I gave away. This went on for 3 years until I decided those Comets were way too prolific to handle,so I gave them away and switched to Koi. The most I’ve had in my 800 gal. pond were 3 Koi (until the Blue Heron found us…). I now have 1 Koi and 1 Comet who are perfectly happy being together….and no more baby fish to deal with!

  10. I have a question – I knew that this would be the year when I would not want any more fish in my pond – the current number of 13 is just enough. Because I have it netted, the goldfish are thriving where they couldn’t survive before. So what can/should I do with new babies in the spring when they hatch? What are we supposed to do with excess goldfish? Mine are all Japanese species – very pretty and sweet. I have never seen this question answered. I know of no one to give them to, so I hope you have a solution. Please!!!

    • Debbie,

      Like Janet mentioned above, if they are koi, they will thin out a majority of them by eating them. If you have no one to give them to and you do keep them, my suggestion is to look at your filtration system and make sure if can handle the extra load.

  11. I thought the fish would regulate themselves by eating the babies and only growing to the size of the garden. If the fish overpopulate the pond, what do you do with them?

    • Janet,

      Koi will eat some of their babies so you are right about that but their growth is not stunt by the size of the environment they are in like some aquarium fish are. When they do overpopulate, my suggestion is to give them away to friends!

      Thanks for the comment

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