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How to Deal with Green Fish Tank Water

Learn how to deal with fish tank water that has turned green from aquarium expert Joseph Caparatta in this Howcast video.


If you have green water it means you have free-floating, suspended algae in your water. I'll tell you how to fix it. And then I'll tell you how to prevent it from happening.

If you go to your fish store they sell algicides. They're either in pill or liquid form. These will help to kill the algae that's been floating in the aquarium.

Keep in mind that the same thing that will kill the algae will also kill good algae like, you know, stuff that's been growing on the rocks that fish are consuming and plants. So a lot of the chemicals that you find will be toxic to plants in the aquarium.

I like a more, like, homeopathic approach. Dimming the lighting, doing some water changes, cleaning the filter, adding some beneficial bacteria to the tank. And basically starving the green algae.

It's not harmful to the fish. The fish don't mind living in green water. In fact, most lakes and ponds the water is naturally green. So the fish are okay.

But if the tank is in, you know, a prominent spot in your house and you're having guests over or it's in a waiting room in an office, obviously you need a quick, you know, rapid fix to this. And using algicides or, even better, using a UV sterilizer would be the best way.

Some fish stores loan out UV sterilizes. You buy it, it comes with a pump, it hangs on your tank or it's submersible. And within a few days your water will be crystal clear.

You can also use a diatom filter to filter out green algae in the water.

But the best thing is prevention. You know, using the pills and dimming the lights and doing water changes will get rid of the green water. But, the goal is to keep it from coming back.

And, whatever caused it the first time is probably going to keep causing it in your particular aquarium.

So, what causes green algae? Green algae growing in the water column means you have an excess of nutrients and/or lighting.

So, most people that put their tanks directly in sunlight in a window are going to get green water. Unless their aquarium keeping practices and their filtration is kept really clean and they don't overfeed.

You're going to get green water just because there's so much light energy hitting that water.

You also don't want to overcrowd and overfeed. Because, when you have a lot of waste in the water you're asking algae to grow. The algae, there's so much nutrients and so much energy available for growth that the algae no longer needs a substrate to (??), it can grow directly in the water column. That's how much energy is being pumped into the aquarium.

So, minimize direct sunlight. Try to, if you can, move the aquarium away from a window. And, also, don't leave your lights on 24/7. This is another main contributor to green water.

People use the aquarium as a night light. So the tank is getting indirect room lighting during the day or the aquarium light during the day. And then at night they leave it on as well.

If you are forgetful and you travel a lot put the light on a timer. Give it a set photo period of eight to ten hours. Or even 12 hours if you want.

But have darkness at night. That will prevent the water from turning green. And keep the water column clean. You know, do your water changes. Add bacteria to the tank. And monitor your nitrates and phosphates.

If you keep those levels low it's going to be a lot harder for algae to take root in the aquarium.

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