Too Many Fish?

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It’s nearly a consensus among pond experts.

When they are asked to evaluate a backyard pond with algae problems, more often than not they track the root of the problem back to fish in the pond. Fish may not be the primary cause in every case of pond algae, but their influence on the pond is always something to consider.

Now keep in mind, what I’m talking about here in general is the smaller backyard pond. Any small pond is like the proverbial fish bowl. They only hold so much water, and they’ll safely hold only so many fish.

In the simplest terms, fish eat, they digest what they can, and then they produce waste. And like any waste material, their donation makes good “fertilizer” for plants. It’s really part of the natural balance of things when you look at it. Good, desirable plants, live off waste material of fish and as they do, they clean the pond, and balance nutrient levels out too. This keeps ammonia levels, among other things, in a reasonable range which is a great aid to the fish.

There’s only one problem with all of this. Algae is a plant too. And it loves fish waste, decayed food, or organic matter of any kind that’s breaking down into an algae edible form.

It may seem strange to think of algae as a pond balancer, but that’s exactly what it is. It is working to clean up some imbalance (most often nutrient based) in the water. For all of it’s good deeds, this doesn’t mean we have to like it any more than we do, but it might help keep the whole battle with algae in perspective.

Getting back to the fish though.

If you have too many fish in a pond for it’s overall size, then often no matter what you do, it will be hard to keep algae totally at bay. You end up fighting a losing battle because it’s very likely that your filtration system might be overwhelmed, and other desired plants, while helpful, may just not be able to do enough.

So here’s a rule of thumb when it comes to fish in a small pond. Be sure to allocate 10 gallons of water for every inch of fish (and this includes all fish except Koi). For Koi you’ll want to allocate about 25 gallons of water for each inch of fish.

This means that if you have a 1500 gallon pond, this can support up to about 60 inches of Koi. Anything over this amount, and they could be adding significantly to an existing algae problem.

Be sure to try to stay under this ratio to ensure a healthy fish population for your pond size. By doing this, adding desired plants, supplementing with beneficial bacteria, and making sure your filtration systems are operating well, you’ll likely stay ahead of most algae problems before they develop.

One final note on fish. Be sure to not feed them too much food. If you find that a good amount of food is going uneaten and sinking to the bottom, slow down in amount or frequency. One rule of thumb is feed them no more than they can consume in about five minutes time, once per day. This will minimize uneaten food from building up and decaying at the bottom of the pond

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15 thoughts on “Too Many Fish?”

  1. That was an interesting article about How many fish are too many. I never realized that too many fish could be harmful. Thank You

  2. Hi BJ,
    As the article points out it’s best to go by the size of the fish. When we talk about total inches of fish, it’s basically an estimate of the size of the fish in inches, and the total number of fish in the pond.

    So if you had, say, 10 fish and on average these were around 6 to 8 inches in length, you could calculate this out to be around 70 total inches of fish in the pond.

    If your pond was only 500 gallons, this would tend to only support about 50 total inches of fish so you’d be overstocked in this case.

    This is all just a general rule of thumb and may depend on the quality of your filtration, how many plants you have in the pond, if you supplement with beneficial bacteria, etc.

    But fish influence is certainly one of the main areas to check on if you have algae issues.

    Take care,

  3. This is very interesting to know. I have about +- 70 koi fish and i think its too much for the size of the fish pond i have.

    Thank you

  4. Thanks for the information you sent tha must be one of my problem. my pond is about 500 to 550 gals and I hwve about 13 goldfish, water stays green all the time. thanks again

  5. Mark,
    HELP! My fish were busy! With the warmer weather this week, the top layer of ice melted and there were at least 24, 1-2″ fish as well as my original 4 or 5. My pond is only 125 gallons. I know it’s too many but do I get rid of them now or wait till spring?

  6. My pond is about 200′ X 150′ and has an average depth of 5′.I have no real accurate way of measuring the number of fish but I do know there are 15-20 White Amure (type of Carp)w/an average lenght of 25″. There are also somewhere between 20 -50 catfish that vary in lenght from 6′ to 16′. There are a few Largemouth Bass but not sure how to even estimate how many. We did catch on this past year that was 21′. of course there are some miscellaneous small fish. I have tried planting water lilies and they flourish for about a year and then they “disappear”. Even the roots are gone. (?) I do have an areator and do have some algea problem. I did use your “Aquaspheres” last year and was able to keep the pond relative clear. It does have an odor and is cetainly not something that you could swim in. The oond is totally Spring fed. Ant ideas?

  7. Hi Glenn,

    Usually with larger ponds environmental factors play a great role than the fish do. This is not to say that a fish population couldn’t get too dense in a pond like yours but obviously there is a lot more room to play with.

    If you have an odor coming off the pond, that’s usually indicative of a build up of decomposing organic matter at the bottom of the pond. Most often this is leaf debris or other matter that’s now turned rotten. I would suggest this is the root of your algae problem.

    Aeration is probably helping as did the spheres. So much depends on how much build up is there. If it’s a lot, it may take time for this to lessen with periodic use of beneficial bacteria and aeration.

    One thing you wouldn’t want to do of course is to kill existing algae off with a chemical. Then it will just sink to the bottom and add to the decay.

  8. Hi Mary,
    From a pond management perspective I would probably try to lessen the fish load sooner rather than later. But ideally it would be nice if you knew someone who could take some of these extra fish on with a pond start up or something, and it might be a tough time of year for you to do that depending on where you live.

    I would at least start thinking about how you want to manage this before things warm up where you are. It will help keep the fish that you do have healthier and happier.

  9. Hi Mark

    Right you are on the amount of fish contributing to algae problems. One thing pond owners forget to consider is also the future growth of the fish in their water fish ratio.
    Ponds should only hold half the allowed fish lenth/water ratio to allow for the future fish growth. Koi can often grow 1″ to 2″ each year in most regions and more in the warmer regions.

  10. Hi Christine,

    Much will depend on how large your fish are in inches. 12 Koi isn’t necessarily a lot but again, how big have they gotten, and how much bigger might they get. That would be the important question in terms of whether your pond can provide a comfortable home for them.

    If you have an algae issue, again, fish can have an effect. They may not be the only thing but they need to be considered. Work as best you can to keep a balance in the pond using plants (if possible), beneficial bacteria, and good filtration. That’s the ideal way to keep algae in check.

    And thank you Mark…you’re correct, growth must be taken into account with fish. Just because you may have the same amount as last year doesn’t mean their influence can’t increase through growing.

  11. We have a 15,000 gal. pond with 7 tiers of falls. The falls are not run constantly as to the expense of running it. However, we do run irration constantly including the winter months and have had a problem with algae (string) in the summer killing some of the fish. We have about 20 koi and goldfish no larger than 6 inches. It is a challenge to keep it clean in the summer months. Any suggestions?
    Cindy in RI

  12. Hi Cindy,
    I’ll throw my two cents in here.

    Your pond is large enough to serve as a good home for a lot of fish. Overstock is likely not a problem here. It’s hard to say why you have algae other than to say whatever nutrients that are supporting it’s growth simply aren’t being removed or balanced out.

    Fish die off is always a concern, and it’s not a bad idea to check on oxygen levels and ammonia levels just to see where these are.

    With algae though, I would make sure the filter system you have is adequate for the job at hand. Something easier to work with is supplementation with some beneficial bacteria at the start of every season. And particularly in the summer, you should try to add some helpful plants to the pond. Floaters that can help shade the pond and consume some of the nutrients that algae would feed on, might help.

    Koi can sometimes get to eating at these, but it’s worth a try to see if you can bring about a balance to the pond. Once you’re able to do this, algae will likely reduce or go away completely on it’s own.

    Just a few ideas to pursue as we move into spring.

  13. Hi Mark. We have a 3 year old 4500 gallon pond with two waterfalls. We started out with 4 koi and added 2 more last year…but we also got a few goldfish/koi from someone who was moving. They multiplied like crazy and now we probably have 35 fish. Now that winter is over, I see even more tiny ones!! But I also see a lot of hair algae already, and we don’t even have the filtration system working yet. Your suggestions PLEASE!!

  14. Hi Sandy,
    A couple of things stand out in your post. I would definitely keep an eye on the fish load here and if needed, be sure to remove what’s necessary to keep the pond balanced. The suggested ratio of fish per gallons is noted in the article above.

    Also, your filtration system will certainly help a great deal once that’s going. Same thing with desired plants. Often in the spring, these aspects of pond management aren’t in fill swing yet, while the fish may already be increasing nutrient loads.

    As the desired plants come along and get into full bloom, and your filtration is up and running full steam, those will surely help with the balance.

    Keep in mind, these beneficial things, along with other steps still may not overcome an issue of overstocked fish for the pond size. At some point, there’s no way around the fact that a pond of 4500 gallons will only hold so many fish safely…so that’s always a key thing to keep an eye on.

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