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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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