Can You Spot Treat Pond Algae?

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Filamentous algae06

Here’s another good question from the old Pond Q&A mailbag and I think it can be applied to just about any size of pond, even though in this particular case the question was about a 3 acre pond.

The gentleman described his situation as having some reasonably heavy string algae around the perimeter of the pond and wondered what he could do about it.  He wanted me to put all the options on the table so to speak and so I did that, and I admit, I hedged a bit on the fact that the pond was so big and that if costs where a consideration, I’d try to include some reasonably affordable options too.

After sending the email out however I began to realize that I may not have talked about this much on the blog and so today, it’s a good opportunity to cover it.

So, when is it a good idea to spot treat an algae bloom?  When does it makes sense, versus treating the entire pond? Or do you always have to treat all of the pond to get a good effect?

And my answer, definitive as it may be is…it depends.

In the case of the initial question, where the pond was 3 acres in size and had algae only around the edges I don’t have as much of a problem suggesting that you target this directly with some kind of treatment.

I would focus first on using some peroxide based algaecide that can be broadcast on this wet algae.  Since it’s a contact type of algaecide, once wet, the pellets will fizz and bubble usually and it’s through t his action that it damages the algae and will kill what it contacts.  Application rates can vary for this stuff but it’s a good short term spot control option, particularly where you’ve used beneficial bacteria before hand.

Peroxide in the water is very short-lived.  It goes inert very quickly and then turns to oxygen and water.  But it will damage many algae that it comes in contact with through a caustic, or burning reaction.  

Two of the most commonly branded products include Algae Off and Green Clean.

Another type of targeted, topical option is one of the more traditional copper based algaecides.  Earthtec is the one we use, but Cutrine which is very common and widely available, will work as well.  These are usually sprayed on the algae directly and it will kill it pretty quickly in most cases.  The main reason I’m not totally opposed to this use is that if the size of the pond is very large, and algae is very limited, then the concentration of copper going into the pond water will be very low and limited.  It won’t affect naturally occurring beneficial bacteria very much at all.

Ponds with heavier algae infestations, or smaller ponds with less water volume would be a different story though.  It should always be understood that any time you add a copper algaecide to a pond, in some way the natural eco-system of balancing will be affected.  But if you can target a limited algae bloom with an algaecide and limit it’s use, then it’s reasonable to try it.

We’re Talking About String Algae Here

Keep in mind that we’re talking about string or filamentous algae here.  It would be a mistake, I believe, to try and treat green water in any kind of a “spot control” way.  The density of single cells that create green water in a pond simply don’t allow for this targeted approach and therefore when green water is present, you pretty much have to treat or dose according to the full size of the pond.

I generally don’t advise or use the peroxide algaecides for green water as well.  It’s not that it won’t help sometimes but because the treatment is so short-lived, and single cell algae can grow so quickly that green water will usually come back pretty quickly and in my mind that just isn’t cost effective if you have to apply the stuff every few days.

What About Natural Treatments?

Biologically speaking, most folks know that I prefer to use beneficial bacteria in a pond to try and clean things up enough to limit any unwanted growth.  It’s indirect of course because these microbes don’t kill algae or anything, they just work to lower or lock up nutrients so much that algae can’t bloom out very well.

When using a good bacteria though, you simply have to treat the full gallon volume of the pond for the best effect.  There’s no way to really treat just a part of the water because then the microbes get too diluted and you’re actually underdosing…which is very good with bacteria.

The one caveat I would make with bacteria based products would be in regard to muck reduction pellets.  These can be targeted very well and treat only certain spots in a pond.  That’s because they actually sink down in the mucky bottom and begin to break this organic build up down.  Often as the bottom get’s cleaner, the surface will improve too.

What About Small Ponds?

As I mentioned earlier, I don’t usually suggest using much copper in smaller ponds.  If you apply too much too fast, or the algae bloom is widespread, you can run into anything from a toxic build up of algae to a massive reduction of oxygen because of a rapid algae die-off from the chemical treatment.

This is not to say that spot control can’t be done in a small pond.  What we use specifically for this is the peroxide based products mentioned above.  Something like Algae Off can be applied right on string algae, or algae that’s growing on the waterfall area, rocks, or stream beds, and it works well in those particular settings.

And as we mentioned with the bacterial pellets, the same can be done in a smaller pond.  If your goal is to reduce muck either throughout the pond or just in certain problem areas, the pellets lend themselves to that kind of application.

These are the most common ways that we might try to use spot treatments for various types of pond algae control but I may not have covered them all.  Do you have any examples to share or any other questions about keeping algae down in your pond?  If so, please post those comments and questions below.


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