Summertime Fish Kills And How To Avoid Them

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As a pond algae specialist (we all have to grow up to be something right?:) I make a point to keep tabs on the various happenings around the U.S. on algae and came across this article on a recent fish kill in a community pond.

This is just one incident that got publicized somewhat, where many others simply go unreported, but let’s just say that mid-summer fish kills aren’t all that uncommon.

High temperatures are largely to blame because once you see air temps getting into the high 80’s and beyond, it’s a fairly good bet that in some ponds the water temperature will be near or above 78 degrees and that’s kind of a magic number. Higher than this and the water’s ability to hold on to dissolved oxygen will begin to drop and DO is critical to fish health.

Signs Of Low Dissolved Oxygen In A Pond

One of the first indicators that you may see when DO is low is that the fish will start gasping at or near the surface for air. They’re not really pulling oxygen out of the air. They need it dissolved in the water to use it, but when you see this activity they are trying to get into this very small sliver of a line of air running just below the water’s surface. In stagnant ponds that’s about the only place they can pick up any DO and sometimes it’s not enough.

If high temps are maintained with nothing like rainfall or cooler weather, then often it will be the largest fish that go first. They have a higher demand for DO due to their size and will be the first to succumb to the lack of it. Ultimately though any fish could suffer the effects of oxygen deprivation and the outcome usually isn’t good.

Certain species like trout are ultra sensitive to many things and low oxygen is one of the big ones.

So What Can Be Done?

I can sum up the best defense, or corrective measure against low DO.


In the end, I don’t care if it comes from a good rain, a fountain, spraying water on the surface of small pond, or using the preferred submerged aeration…in a time of crisis, anything is better than nothing at all.

But you may notice that in the article they stated that the fountain was working. And this is likely so. But if this pond has much depth to it…let’s say over six feet or more, then a fountain has some limitations on how much of the pond it will actually aerate. Normally fountains will help in the upper few feet of the water, but they generally don’t provide the full degree of aeration that submerged diffusers can.

Also, it was noted that algae may have played a part in the die off of fish. This certainly could aid in lowering oxygen in the water because as algae grows, it releases DO. As it dies off it will pull DO and coupled with high heat it could deplete oxygen really quickly.

So the second lesson in the battle against low DO is to try and keep the pond cleaner and clearer of any blooms or unwanted growth. This includes algae of course but also weeds of various kinds. A pond doesn’t have to be completely free of growth and a small pond, as we note often, will benefit greatly from some plant life. But prudence will pay to keep things managed during the really hot and stagnant times of the year.

In an ideal world, a pond owner would see this problem coming. I mean summers are usually fairly hot around most of the U.S. But like me, if you don’t perceive much of a problem you probably won’t do anything about it until bad luck strikes or you become aware of the consequences. We normally get a huge percentage of our calls on aeration during the hottest parts of the year, and sometimes this correspondence comes a bit too late for some fish.

This isn’t meant to be, nor should it be taken as a criticism. It’s simply an observation that we might be able to learn from. And the lesson that I’ve learned through hard experience is get ahead of the heat. Have a well sized aerator running 24/7…and yes that means all day everyday, before things really get hot. Try to avoid getting too conservative just to save a few bucks on the electrical bill during the really stressful times if you can afford to do that. It will pay good dividends in the end.

Do the best you can to keep growth of various things under control. It’s best to attempt to do this early in the season as algae or weeds are starting to pop up. I generally don’t have too much of a problem in using a good pond bacteria at any time of year, with good aeration. Algaecides and herbicides require a bit more discretion in my book and unless you can treat spots or areas, one at a time over a period of time, I would forgo any major pond wide treatments to kill everything until things cool down.

Any Pond Can Be Affected

I think there is often a prevailing myth that fish kills like the one mentioned above will only happen in larger ponds. Of course this is the kind of setting that you’ll normally hear about in the news. But small ponds can be affected as well.

For anyone that’s ever visited with me on the phone in a consultation, one of the first questions I’ll ask is, “do you have an aerator running?” And a common answer from a small ponder will be, “well, we have a waterfall.”

I’ll be the first to admit that this can only help. Any movement, any splashing, or breaking of the surface tension on the water will be useful. But it may not be enough during really tough stretches of weather, particularly if the pond is spread out a little bit. It’s not uncommon to see your fish hanging around the falls area during hot weather, and when they do it’s usually indicative of lower DO in other parts of the pond.

So regardless of one’s pond size, aeration has it’s merits.

I think it’s a bit of stretch to say that of the thousands of customers we’ve worked with, and many of those either have an aerator, or were convinced to finally get one into action, there will be very few who don’t believe that they make a world of difference in the quality and health of their pond. A pond aerator won’t necessarily be a cure-all for everything, but pound for pound, it’s an extremely useful and powerful tool for the pond owner.

If high heat and stagnant conditions are common in your pond during the summer months, at least now you know where to start your research and education on improving things for the seasons to come.

One other tip I’ll leave with you today…and the video below will help. If you’re trying to play catch-up and get aeration in place during a hot period, this information will prove to be important for you. Watch the video to learn more.


For more detailed information on the aeration systems we have a available please visit the link below.

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