Do Algaecides Kill Fish?

If you have had a water feature for any amount of time, you have likely experienced algae blooms. For many this unsightly little green nuisance can be a constant battle. Many people will simply add an algaecide. Some of those people however, find even more problems in this solution. Perhaps you have even experienced it yourself but after a dose with an algaecide, you lose fish. Quickly it is easy to blame the algaecide, it killed your beloved fish. Does it really though? The answer actually isn't quite so simple. Of course one factor is specifically what algaecide we are using. There are some that simply aren’t meant for use in our fish ponds. But for this discussion we are focused on the “fish safe” algaecides. Most of the algaecides for ponds on the market today share the same active ingredient; Poly(oxyethylene(dimethyliminio)ethylene(dimethyliminio)ethylene dichloride.

That’s a mouthful for sure, so we’ll use the term “polyquat”.

Again there are many brand names of algaecides using polyquats as the main ingredients. There may be slight variations in the concentration or what the inactive ingredients are, but they are more or less the same.

Do these polyquats kill fish? Short answer is no. When used as directed all of these algaecides should be considered fish safe. But, we still hear horror stories of fish kills after using one of these treatments. The first thing we need to understand is the algaecide itself is not killing the fish, but there absolutely can be a direct correlation between their use and the death of fish. The sudden removal and decomposition of algae consumes large amounts of oxygen. So, when there is a fish kill after using a polyquat algaecide, the chemical didn’t kill the fish, the lack of adequate dissolved oxygen did.

At first this may seem like semantics. Regardless of HOW the fish died, it was a result of using the algaecide. This is true, but understanding exactly what happens allows us the options of safely using the treatments if desired.


There are 2 main reasons the oxygen level will be depleted to dangerous levels when using the algaecide.

1. There was barely enough oxygen to begin with. If the pond had too many fish or just simply was not well-aerated then the fish were walking a fine line…err…swimming a fine line already and all it took was a minor change to the dissolved oxygen levels to start killing fish. The solution here is relatively self-explanatory; maintain a healthy fish load and ensure adequate aeration. While a simple waterfall may be adequate aeration, supplementing with an air pump is almost always advisable. And whatever aeration you are running, make sure it is operating 24/7.

2. Overdose of the algaecide. Overdoses are by far the most common reason fish kills occur after algaecide treatments. This can be as simple as mis-measuring the product, but it is almost always over-estimating pond volume. Most people do not know the actual volume of their pond, they have estimated based on inaccurate measurements. This is because most ponds are not a perfect rectangle with a uniform depth, so volume calculations are estimates at best. There are ways to know the pond volume, but when it is an estimate you have to be VERY cautious with your dosage of treatments.

Algaecides can be an effective treatment to alleviate some algae in our water gardens, but it does require taking proper precautions. There are also many other effective means of algae control that may be a little less risky to our fish.