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Pond Algaecides – Usage and Precautions

Hey guys, Justin here! Its about that time of year when your pond may be turning into something resembling large vat of pea soup. The water turns green, the liner turns green, and thick green strings of algae grow on everything! It’s painful to see, because you work so hard on your pond to keep it nice, clean, and clear. Though I recommend using algaecides only as a last resort, it is very important to ensure that you are using them correctly if you choose to go that route.
The first thing to know when treating your pond is the size of you pond in gallons. This is imperative, as over dosing algaecides can kill ornamental plants, frogs, snails, and even fish. Under dosing, while not harmful, is wasteful and will obviousle make the product less effective in combatting the algae. If you are not sure of how many gallons your pond holds, there are many formulas to help you determine the volume. Rectangular ponds use the formula Length x Width x Average Depth x 7.5,  Circlular ponds Width x Depth x 70.5. Make sure that all of your measurements are in feet. Oddly shaped or angular ponds may be a little tricky, but generally you want to calculate using the surface area x the average depth. Once you have the volume of your pond calculated, simply follow the directions based on the manufacturers suggested doses.
Once you have added the algaecide to the pond, it is important to remember the last half of algaecide is “cide”. This means that algae will in fact die, and those of us that have kept ponds and aquariums know that dead organisms left in water start to break down. As the dead algae is being broken down by bacteria in the water, Ammonia and CO2 will be released into the water. Ammonia is very toxic to fish even at low levels. It is important to add bacterial additives to your water in conjunction with algaecides. The additional bacterial additives will convert the excess Ammonia to less toxic nitrite, then again into nitrate.
As the bacteria are breakdown the dead algae, they also release CO2. As the CO2 is released, it will actually force the available oxygen out of the water. When treating your pond with an algaecide, it is very important to have adequate aeration and surface agitation to supply your fish with vital oxygen. This is especially important during the warmer water months, as warmer water tends to hold less oxygen than cooler water. CO2 build-up in conjunction with the demize of oxygen contributing (and unsightly) algae, will cause an instant drop in available dissolved oxygen! Be aware of treating in the evening, too. Oxygenating plants will not produce as much oxygen in the evening and through the night. Even if you have bunches of oxygenating plants in the pond besides the algae, fish may still suffocate if the pond is treated late in the day. It is best to treat early in the day when you are able to observe how your animals are responding and take steps if necessary to add more aeration. (Low oxygen levels may cause fish to become sluggish, listless, or cause them to gasp at the surface or congregate at a fountain head or waterfall where more oxygen is available.)
Remove dead and decaying algae as much as possible, before it begins to cause other problems. Once the algaecide has run its course, it is recommended that you do a water change to remove the remaining algaecide and free floating algar, and vacuum the pond bottom to remove the debris. There are also a variety of “sludge removers” to help breakdown the remaining debris before problems begin.
Following these simple steps and precautions will ensure that your pond remains clean and clear and help you to avoid any algaecide caused fatalities (except the nasty green invader). Good Luck and happy pond keeping!


  1. avatar

    When the water of the pond turns into green, it is the best time to leave some fish there and you will notice that fish in the pond are having growth very nicely.

  2. avatar

    My wife put algaecide from the pool into our large fish pond (~10×15′) and killed all the fish. Wondering how long we need to wait for this to breakdown, or if we need to do a complete water change?

  3. avatar

    Mike, what product did you use? Many algicides are oxidizers, and a common problem is that they can deplete the dissolved oxygen on a pond for a short period after dosing. It’s important to keep the pond aerated and to treat during the day when oxygen levels are higher. Treating in the evening can cause big problems for fish, causing them to gasp for air at the surface or to mass together at fountains or waterfalls where the water is being aerated. It’s very important to read directions and warnings thouroughly before any chemical treatment to prevent sad incidences. Depending on the product, the effects only last for a short time. A bag of carbon can help to remove residual treatment and partial water change/vacuuming of the pond bottom can help to remove debris and organics to prevent future algae outbreaks.

  4. avatar

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  5. avatar

    Very informative post and guiding about the precautions is really required because people used to ignore this. As precaution Liquid Form EPDM Pond Liners is used and stop the algae growth in pond with curing leaks and keeping pond fresh.

  6. avatar

    Today I over poured algiside, I did a water change pumping water out putting clear water into pond. 4 fish died. I removed 8 remaining fish that I could find and put the 8 in a fish tank with 2 bubblers. Any other things I can do to save the rest of the fish I couldn’t get out. I had 20 fish,4koi

  7. avatar

    Hi Linda, What product did you use? Like the previous commenter that had this same problem, the solution would depend on what you used. In general, a partial water change, some carbon in the filter to remove any chemicals and extra aeration to keep the oxygen levels up will help.

  8. avatar

    I have done the same, used Aquarius Pool Algaecide in my pond, it killed all the frogs, I removed any access to it because song birds and chipmunks also bathed and drank from it.

    Is there anyway to bring my pond back to nature ?
    thanks in advance

  9. avatar

    Hello Ryan, I’m not sure what you mean by “back to nature”. If you are trying to remove the algaecide, you can add carbon to your filter and it will pull out any remaining chemicals in the water. Since the product you used appears to be a swimming pool algaecide rather than a pond algaecide, we don’t have any information on it. I would recommend contacting the manufacturer for more information on the best way to completely remove it.

  10. avatar

    My son pour algaecide in our pond and killed most of our fish. We dont have any algae. How long do i wait to put more fish in the pond? Also is there any way i can remove the algaecide?

  11. avatar

    Hi Elias, What size is your pond? Is it a natural bottom or is it an artificial pond with a liner? What type of algaecide was used? Chemical filtration like carbon should help remove most chemicals but it is difficult to say if it would work for you without more information. Since you had a big loss, you will need to test the water to make sure it is healthy enough for the fish you have left and any new fish. Ammonia and Nitrite should both be zero and pH should be consistent and around 7.0-8.0. If you would like to speak with someone here so we can address your situation more specifically, you can reach our store at 717-299-5691.

  12. avatar

    i have a 5,800 gallon koi pond for 15 years. No issue until two weeks ago i accidently double dosed the pond with Algecide. Ive lost all but two koi. Done three major water changes. All other levels test normal but obviously the water is still toxic. How do i remove the residual algecide from the pond water and how do i test the water before adding new fish?

  13. avatar

    Hi Ray, Do you have any filters on the pond? Adding carbon or a chemical filtration pad like PolyFilter to the filter or in an area of high flow will help to remove any remaining chemicals from the water.

  14. avatar

    Hi I have a big pond, meaning it is 150 feet by 100 feet and average depth is 6 feet except along the dam which is 15 feet deep. I noticed vultures yesterday and something white which I thought was a plastic bag. Today all the neighborhood vultures were hanging out in the backyard. Tonight I walked out and found 45 dead fish, catfish, bluegill, and bass. Algaecide was added 3 weeks ago, possibly 2 weeks ago. I thought if too much was added the fish would die relatively quick, maybe 2-3 weeks is quick. I read other posts about the breakdown of algae and plants creating ammonia and releasing C02. I just turned on the fountain and am now greatly regret having the lawn man adding algaecide. Prior to him treating the pond, I had been running the fountain for four or five days in a row. Things seemed to improve. The lawnman has treated the pond for maybe last five years. He presented a receipt for algaecide and it was around $200.00. Had I known the expense involved I would have passed on treatment, especially since the fountain seemed to help. This is my dad’s pond, he is in assisted living and it’s my first year managing things. I hate failing at things. Also if I introduce Lily pad starts like the ones sold at Lowe’s is this going to be something else I regret? Fortunately the algaecide killed them so no worries now. Any advice? Should I be scooping the dead fish out? All but 1 is along the shoreline. I live in Illinois and we had a fair amount of rain this summer. Thought that was noteworthy. How many hours a week should the fountain run? 4 or 5 days running 6-8 hours seemed to add $50.00 to the power bill. Thank you in advance for your help.

  15. avatar

    Hello Cheryl, I’m sorry to hear about your losses and trouble with your pond. Anything dead should be removed from the pond since its decomposition can harm your water quality and make matters worse. If your fountain is the only source of water flow in the pond, it shouldn’t be turned off, especially if you are using an algacide that can deplete the oxygen from the water. If you have other filtration or circulation and the fountain is merely ornamental, you can run it as much or little as you’d like. For a pond that size, I would recommend consulting a pond expert in your area regarding the algacide used and the amount of die-off you are seeing to see if they can test your water and recommend a treatment plan. We wouldn’t have information specific to your area but you can try starting with this link from the Illinois Department Of Natural Resources to contact someone closer to you.

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