Green water in aquarium: how to fight it in 5 easy steps

Green water… sigh.

I’ve been there myself, a long time ago, back when I was a little boy discovering my budding passion for aquascaping. I started with a small tank with 2 goldfish. Totally wrong, of course, because those creatures shouldn’t be kept in small tanks. Plus, they’re known to be big polluters.

You can probably guess the result… after a week, they were swimming in a completely green tank. You could only see their “shadows” swimming around because the tank was so filthy. I thought, “I’ll just empty all the water, scrub it clean, and everything will be fine.” In hindsight, that was entirely the wrong approach, and on top of that, it didn’t work because in no time, I saw my aquarium water turning green again.

And I’m far from the only one who’s dealt with this. Not long ago, I got a question from a reader who was fed up with this green water. He wasn’t keeping goldfish but had a community tank with several aquarium plants. Nothing’s more frustrating than not being able to enjoy your tank because of that ugly green water in your aquarium.

That’s why I wrote this guide: to easily combat green water in aquariums!

What is green water in my aquarium?

To start: what is green water in aquariums anyway? If you’re dealing with aquarium water turning green, this is caused by a single-celled algae. When light hits your aquarium, it can look like some kind of fluorescent, nuclear-green substance.

Thankfully, if your aquarium water becomes green, there’s something you can do about it.

What causes aquarium water turning green?

The cause of aquarium water turning green can usually be traced to one of the following factors:

• Your aquarium water is too warm
• Your aquarium is too brightly lit
• There’s too much waste in the water (especially with goldfish!)
• A dead fish in the tank is causing an ammonia spike
• Your aquarium filter isn’t working properly, so waste isn’t being processed effectively by bacteria

Poor plant growth can also lead to green water in aquariums. Plants that don’t grow well create waste instead of processing it as healthy plants do.

These factors can also contribute:

• Not using enough nutrients, which can cause plants to grow poorly and produce more waste
• Improper CO2 levels in your aquarium, leading to poor plant growth and waste production

How to prevent green water in your aquarium

As always, the best tactic is to prevent aquarium water from turning green. But how do you do that?

I’ve created this handy checklist against green water to help you take step-by-step action to prevent green water in your tank.

1. The first question to ask: is your tank in the wrong spot? If your aquarium is constantly in the sun or in a very warm spot, you’re asking for trouble and for aquarium water turning green. Place your tank somewhere protected from direct sunlight and ideally in a calm location.
2. Is your aquarium too small? If so, you’re likely to face aquarium water turning green quickly, as the fish won’t have enough space. Fish produce a lot of waste, and before you know it, you’ll have green, cloudy water in your aquarium. Plus, it’s much better for your fish if they have enough space. You’ll find it not only helps prevent green water in your aquarium but also keeps your fish and other aquarium inhabitants healthier. For goldfish, for example, aim for at least 50 liters of water per fish.
3. Are the first two steps sorted? Then check your aquarium filter. If you bought your tank as part of an aquarium kit, the filter might be underpowered. If your tank is full of plants or fish, you’ll definitely need a strong filter. My advice? Choose a filter with a minimum flow rate of 5 to 10 times the volume of your tank to ensure good water circulation in your aquarium. For a 50-liter aquarium, that means a filter capacity of 250 to 500 liters per hour.
4. Think about the balance between CO2 and plant nutrients. Healthy plants not only keep your tank clean, but they also look better and produce less waste. Three great reasons to focus on this!
5. Besides avoiding direct sunlight, be mindful of your aquarium lighting. Strong aquarium lighting is great for plant growth but increases nutrient demand. If you don’t meet that demand, trouble arises. Your plants stop growing and algae take over, including green water!

With these five steps, you can almost certainly prevent green water in your aquarium, or at least greatly reduce the risk. Do you already have green cloudy water in your aquarium? The steps below will help you get rid of it quickly.

How to deal with existing green water in your aquarium

Follow this simple guide to make your cloudy green water aquarium crystal clear again. Once it’s resolved, you can follow my prevention steps to keep it that way.

Easy peasy!

1. Start with the basics: a good water change! Check out my tips for quick and efficient aquarium water changes. If the green water in your aquarium is severe, you might need to do several water changes in a row. Be careful, though! Make sure the new water matches your tank’s usual parameters to avoid stressing your fish and plants. Check and note the pH, KH, and GH, and add minerals during the water change if needed.
2. Eliminate the last traces of green water in your aquarium with a UV filter. UV light from a filter directly attacks algae spores. I personally don’t use a UV filter for longer than necessary—usually around two weeks. Using it permanently can affect some components of plant nutrients, making them less stable.

   

3. While the UV filter works on the algae, continue weekly water changes and, of course, maintain your plants and clean your aquarium as you normally would: optimize CO2, add plant nutrients, care for your fish, check your filter, and keep other equipment in good shape.
4. Finally, after about two weeks of using the UV filter, the algae should be dead. Just to be safe, do one more thorough water change. That way, you can rest easy and wake up to a sparkling clean tank!

Conclusion

Fighting green water in your aquarium with this guide is totally manageable. Actually, it’s mostly about preventing your aquarium water from turning green by sticking to a few basic rules:

• Avoid waste buildup
• Perform regular maintenance
• Keep your plants in top condition

If things go wrong, or you’re reading this because you already have green water in your aquarium, just follow my guide to get rid of it. Also, be sure to check out my e-books for preventing algae.