Effective Solutions For Green Water In Small Ponds

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Written By Mark Washburn

Mark has 20 years of experience as a professional pond management consultant.

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Green water in a small pond can be one of the most frustrating experiences for the pond owner.

Once established, and you should know, it doesn’t take very long for that to happen, some folks end up trying all sorts of remedies and potions, without seeing any positive results.

So before we dig into the best ways (I think) to deal with green water problems, just know you’re not alone. It’s a common problem that may have any number of solutions that can end it.  I’ve included a video here where I’ll cover the steps that we take to deal with green water problems in a small pond.

For anyone that has ever filled a pond with water, you know that it’s very possible for green water to show up in a matter of days, and then hang on for weeks before it starts to clear up. As I mentioned, it’s not uncommon at all for this to happen, and much of this is due to the pond being new, or newly filled, and the baseline, biological processes haven’t really set up yet. In other words, this fast growing single cell algae is just quicker off the blocks to get going.

Sometimes Green Water Becomes A Chronic Condition

The good news is, in many ponds, if they are healthy and balanced, the green water will start to go away after a few weeks. If you have a good filter, and usually that means some form of biological filter, primed with some good bacteria at the very start of the season, after six to eight weeks, balance is achieved, nutrients in the water drop, and the algae bloom starts to clear up. A pond could go from green, to clear, back to green for a time, and then clear up again eventually, and stay that way through the season.

Or not…

Let’s say though that you’ve done all the things suggested in starting up a new, or newly filled pond.  The bio filters were cleaned initially, then you used some good microbial product to re-establish the beneficial pond microbes in the pond and filter…you’ve got fresh water and a fairly clean bottom, having removed any  muck or sludge and debris from the previous season.

But green water shows up anyway.  And after a month or two, it’s still not going away.  What’s the deal!?

Well, there are times when even with the best intentions, green water will remain stubborn and there are a variety of reasons for that, but I like to focus on why the beneficial bacteria may not be working at their best, because they really are one of the main keys to having a clean and healthy pond.

Beneficial Bacteria Must Be Supported

Some of this may be due to water chemistry.  And this is worth checking out early on.  Is the pH reasonable?  Meaning is it somewhere between 6.8 and 8.5?  What about water hardness and alkalinity?  Ideally these should be around 75 to 150 ppm for hardness, and 120 to 180 ppm for alkalinity.  You can test these parameters with some simple 5 and 1 Test Strips that can be purchased online, or quite often, at your local Walmart in their aquarium/pet section.

Basically these numbers will represent a good supportive environment for the beneficial bacteria to work their best.  Then it’s a good idea to make sure the pond is well aerated in some form, because many of these good microbes benefit from increased oxygen levels in the water.   Covering the bases of good water chemistry and oxygenation will at least maximize the performance of the microbes you put in the pond initially.  And this can often result in a clearer pond…no doubt about it.

My Process For Dealing With Green Water Algae

In cases though where green water continues to plague a pond, I start to work through a process, sometimes stacking various treatments or actions, that are specifically designed to help combat green water.  We must consider the stimulants to this algae growth, and figure out ways to disrupt these supportive elements, ideally without using any harsh chemicals.

As far as supportive elements I think it comes down to just a few things.  Nutrients, sun exposure, and limited filtration.  The steps outlined in the video below will cover ways to deal with these issues, and move eventually into some direct tools and treatments that are solely designed to kill the algae in the pond.  I’ve also added a few additional notes below the video that will address some things I may have missed when I first recorded it a few years ago.

Here’s a few additional comments not mentioned in the video.

A UV light is a very targeted and direct way of affected green water or single cell algae in a small pond.  It was specifically designed for that problem in particular, and won’t be of any help at all to string algae, or any algae that can’t pass through it.

Still despite having a UV installed in a pond, some people still have problems with green water.  There are a couple of main reasons I see that could cause this.

  • Outdated bulbs.  UV bulbs should normally be changed every season, or something around 12 to 14 months of use.  A bulb may still light up but will lose power over time…so if you found good results initially but after a year or so of use things are turning green…it’s probably the bulb.
  • You flow rate is too fast.  A UV should be sized not only for the pond’s gallon volume (which I would view as a baseline) but also for the flow rate of the pump itself.  Many folks like to have good running water falls and to support that you may need a pump that exceeds your gallon volume by a good bit.  If so, the UV needs to be sized to the greater of the two, otherwise the flow of water will be too fast, and latency in the device too short, and this won’t allow the ultraviolet to kill off or damage the algae cells very well.  Either decrease the flow rate, or increase the wattage of the UV.
  • Improper installation.  UV’s are not really hard to install, and when  you look over directions on how to do so, in some cases even the manufacturers directions will vary.  What I mean by that is some advocate putting the UV in line before the filter, and some advocate putting it in after.  As for myself, I like the idea of having the UV installed upline of the filter.  The UV will damage the algae, clumping it together, then the filter can more fully take the algae out of the water.  In my mind, a UV should have a good biofilter installed along with it.

In the video I mentioned a product called Acurel.  We used this for several years and it worked well in many ponds.  However we shifted to another brand of clarifier which was less expensive and has worked just as well.

And finally, I didn’t mention barley based products in the video, but I am not at all opposed to using them.  Most university research indicted that barley can work best as a preventative and so that’s how I like to use it.  Despite that, over the years, we’ve had many people tell us that barley helped to clear up and algae problem in their pond, so the bottom line is, if it works for you, run with it.  It’s safe and fairly affordable.

Products Mentioned In This Article.

Fortunately most of the products mentioned above are easy to find online, and even locally in some cases.  Here’s a few links to those I recommend, and as you might expect, some of the best prices can be found at Amazon.

Beneficial Bacteria

Small Pond Aeration

Ultraviolet Lights For Ponds


Easy Pro Water Clarifier – Similar to Acurel but less expensive.


Barley Treatments

If green water has been troubling your pond of late, there are certainly things you can to do to help clear it up.  It may take a bit of trial and error to see what works best for your situation but hopefully this article and video will give you a few ideas on how to reduce or eliminate this persistent algae problem.

If you have any additional questions be sure to add your comment below and I’ll try to answer them.

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