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Be Aware of Oxygen Levels When Treating for an Abundance of Algae – Pond & Lake Q & A

Picture of Filamentous Algae in a Pond

Q: My pond was almost completely covered in algae earlier this month. With the advice of a local store, I treated my pond with an algaecide. Needless to say I suffered a terrible loss! What killed all of my fish? – Marie of Florida

A: More then likely your fish loss was due to oxygen depletion. It is very rare that an EPA registered “Aquatic Approved” chemical will cause a fish kill.

What causes oxygen depletion?
After a chemical application, algae and aquatic vegetation start to die and begin to decompose. The decomposition process requires great amounts of oxygen and can sometimes, like in Marie’s case, be harmful to fish. The chance of oxygen depletion is much greater when a pond is not maintained on a regular basis or when water temperatures are at their warmest such as the dog days of Summer. The warmer the water, the less oxygen it can retain.

How to treat your pond and keep your fish safe:
We recommend that you treat your pond in sections. The generally rule of the thumb is to split your treatment in to 3 parts or thirds. Treat 1/3 of your pond starting from shoreline working your way towards the middle. Allowing 5-7 days between treatments will greatly reduce, if not completely eliminate the chance of fish kill.

NOTE: Always follow the label rates on the container!

Reduce Oxygen Demand with an Airmax® Aeration System Airmax® Aeration adds oxygen to your pond reducing fish kills, while improving the overall health of your pond and fish.

2 Responses

  1. I live in Ky and have a rock quarry that is about 30×40 and 11 ft deep. In the spring I use an algaecide to control algae, about a cup and a half. This spring I forgot. So on Aug 15 I used a quarter cup of the same algaecide and killed all the fish in my pond. To say the least I was devastated. The damage was done. My questiion is what do I do now about the black water and awful smell. I have dipped out most of the dead fish. I would like to restock but don’t know when. Any help would be greatly appreciated.

    • Hi Peter, thanks for your question. It is possible that the fish kill is a result of a lack of oxygen in the pond. In the Spring, and when water temperatures are cooler, the water will hold more oxygen. During the hot Summer months, the warmer water will hold less oxygen, and to add to the situation, dying weeds and algae will further reduce the oxygen level in the pond. I would introduce an aeration system to your pond. This would increase oxygen while treating your pond, help reduce the dead debris and black water, as well as prepare your pond for a new fish load. In addition, a natural bacteria such as Muck Away will help to further reduce debris and eliminate odors in the pond. Give this a try and let us know your results. Thanks!

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