Does Green Water Hurt Fish?

One of the most common questions we get about algae of any kind, concerns green water and whether this very common problem is actually harmful to fish.

It would make sense to say that this issue of clarity in any size of pond is really annoying to the pond’s owner. What may have once been a pristine and clear body of water, sometimes turns into a pea green soup overnight.

Green water in ponds, in case you don’t know, is actually created by very small, single cell algae that multiply out to the point where the water may become tinted slightly to the other end of the spectrum where you can’t see an inch down into the pond.

Like algae algae, these little fellow love sunlight and they derive a lot of energy from photosynthesis. Some of them even rise and fall during a 24 hour cycle. In the morning and throughout the day, they rise to the surface to soak in some rays, and at night they fall back down to the lower parts of the pond.

Generally speaking, even though they create a very poor looking body of water, they aren’t normally a problem for fish. When a pond get’s a lot of sun and it’s really hot outside, they may even provide a form of shading which could help keep the fish somewhat cooler. This applies to most small ponds and private fish ponds in the U.S.

Harmful Algae Blooms

This is not to say that all algae is necessarily harmless. If you’ve watched the news reports from time to time you’ve no doubt heard of something called “HAB’s” or harmful algae blooms. These are usually made up of very specific forms of algae and they can be considered harmful to humans and marine life.

Specifically you may hear about three primary algae that are lableled as HAB’s. Cyanobacteria or blue-green algae, in high concentrations, can produce various toxins that can make people sick. You have to drink the water or swim in it to be affected. Red Tide is a marine algae that can kill fish and marine animals, and adversely affect people as well. And P. piscicida is another single cell organism that can be found in ponds that will cause fish kills although it’s not clear just how harmful it may be to humans.

What IS Risky For Fish…

For a pond owner, it’s needs to be remembered that it’s usually not the algae bloom itself that’s a big issue for fish. Often the most common reason for die off or loss of fish is the mis-application of a chemical used to kill the algae.

Any plant in the water will add oxygen when it’s thriving and picking up sunlight, and it will pull oxygen when it’s dark or it’s dying. So, when a chemical is used to quickly kill the algae, and it dies off in great numbers, the dissolved oxygen in a pond can be reduced dramatically in a very short amount of time. This, of course, is not very healthy for fish.

So you really have a couple of choices when it comes to treating an algae bloom. You can treat it with a chemical, but if you do, you need be aware of this issue of oxygen depletion. For string algae blooms, it’s easy enough to spot treat these growths in a section at a time. Kill off a bit of algae each day until you get the pond cleared up and be sure to use good aeration during the process.

Green water is a bit more complicated because it’s much harder to target. It’s really best to try to keep the algae from forming in the first place. For small ponds this could mean using a uv clarifier along with a good biofilter to keep the pond in good shape. For large ponds you may decide to use an algaecide, but it should be applied in a way that keeps algae from forming. EarthTec is a great algaecide for this purpose.

Sometimes No Treatment At All Is Fine

The main purpose of this article is to ease the troubled minds of pond owners who may be worried about green water harming their fish. Rest assured that the majority of the time, green water algae will not harm fish populations at all. If a pond has robust aeration in place, you have the best insurance of protecting your fish from a variety of issues and once the weather cools off, the green water will clear once again.

You can certainly try to treat it, and there are effective remedies, but don’t do it for your fish necessarily. They’ll probably do fine even if they are seeing green all around them.

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One Response to Does Green Water Hurt Fish?

  1. Mr Rigg October 10, 2016 at 8:21 am #

    Thank you so much for the imfo. just built a 2000 ltr pond on my patio , filled with tap water
    fish are fine but four inches down ,thats it green, but I will wait for the pond to mature and the plants as well , as you say the fish don’t seem to mind. Again thank you so much .

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