Green water in a pond is one stubborn critter sometimes.
It’s not really unusual to have this type of pond algae crop up and bloom once the days get a little warmer and sun shines a bit brighter in the spring or summer. And unfortunately for the small pond owner who’s plagued by it, their first reaction is often to drain all the water out of the pond, clean things up and then start over. But that’s not usually a good idea because the green water usually comes back very quickly.
Green water is caused by many individual single cell algae of various species. They tend be very quick to bloom and multiply and they respond very favorably to sun exposure. The rays of light really ramp them up.
So I guess the next question is, are there better options than draining the pond and starting over? You bet there is, and we’ll cover a few good ways to deal with this type of algae right here.
First and foremost you have to dig in and think about what’s really stimulating this algae in the first place. It’s always the best place to start.
Like all algae, the plant blooms out and expands in number when conditions are supportive of it. I’ve already mentioned the sun exposure thing and no doubt this is one area that’s good to target for green water. You can’t usually stop the sun from shining, but we can create shade through various floating plants. Lilies are a common favorite but anything that floats and covers some area will be fine. Cut the sun out of the mix and you might make some headway with that step alone.
Algae also feeds very well on nutrients in the water. Nitrates are the most common source and these come from fish waste and other organic substances that decay in the water. So it’s best to keep the pond as clean as possible. Install a good pond filter that biologically addresses the nutrients in the water, and use a good beneficial bacteria to help reduce these as well.
These are fundamental steps that don’t cost much and help in a lot of ponds with single cell algae issues.
While I’m at it, I should mention too that if you see green water early in the season, right after starting the pond up, don’t freak out too much about it. It may take weeks for the biological processes and your good plants to get established so be patient and wait. The tinted water won’t usually hurt anything and you can do more damage by trying the kill the stuff off with a chemical algaecide so try to avoid that, at least early on. Give the pond time to get balanced out. Add your pond bacteria regularly in this early phase and see how things go. Often the pond will stabilize out after 6 to 8 weeks. It may clear up and turn green again several times through this period too so be patient.
What About Chronic Green Water Problems?
If I’ve waited around for the water to clear up and it still hasn’t improved after 8 weeks, I might start to think about doing something more directly to address the problem.
One really simple thing you could try is some partial water changes. It’s more aggressive than the periodic 10% water change that many folks do just as routine maintenance. In this case, you want to change out more of the water, more frequently. Maybe 20 to 25 percent, every few days for a few days or weeks. This is assuming you aren’t in any water restricted area of course.
By doing this you can see if there is a dilution point in the nutrient level. If you get this right, the water will start clearing up and then you know it’s pretty much a nutrient issue. At that point, check your fish loading to make sure you’re not exceeding the ponds ability to hold fish safely. A rough guide that we use is 25 gallons of water for every inch of Koi fish and 10 gallons of water for every inch of Goldfish. If you’re fish load is higher than you gallon size, you might want to lessen the load.
UV Clarifiers and Sterilizers Can Help Too
I’m saving a mechanical option for last here because it doesn’t necessarily have to be the first thing you try in the fight with green water. However, if you’ve gone through a few of the things I mentioned above and still have green water, then a UV pond clarifier or UV sterilizer can make sense. The two terms, clarifier and sterilizer is basically the same thing, or the same type of device. The difference comes in how much gallon volume of water is going through the same uv system. Clarifiers have a high flow rate and allow a bit more through, such as some microbes for instance. A sterilizer will have a slower flow rate and will kill more algae and bugs (bugs meaning bacteria that are both good and bad that reside in the pond).
UV or ultraviolet light, irradiates the water and it’s visitors such as single cell algae as it passes in front of the light. These little plants will be damaged or killed, and usually when this happens they’ll clump together in bigger masses. This allows for better filtering of the debris, so in my opinion, uv will work best along with a good biofilter of some type.
When fitting a UV to a particular pond, be sure to know your pump capacity and the flow rate of the water running through the system. I think the most accurate way to fit one of these systems is by using this flow rate specifically. Ideally you want the full gallon volume of the pond to pass through the uv system, and the filter, about once per hour.
You can also fit the uv light by using a gallon size rating of the pond too, but when you do this it’s probably better to overdo the UV clarifier wattage, rather than cutting the number too close or undersizing. You’ll get better results if you don’t cut any corners here.
In the video below, I discuss UV clarifiers and sterilizers in a bit more detail. If you have any questions or comments about UV and green water please share the comments section below the video.