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How Do I Combat String Algae in my Water Feature? – Water Feature Q & A


Picture of String Algae

Water Feature Q & A

Q: I have a 1,000 gallon pond and already the string algae is starting. I am sick of constantly cleaning it. Any ideas? – Steve of New York

A: Like Steve many of you find yourselves in this same situation, where it seems like you are battling algae year after year with no end in sight. The thing I want you to know is that in order to fully understand how to control algae, you really have to understand how it develops in the first place.

The Key Ingredient:
One of the key ingredients for algae to grow is a food source (aka Nitrates). And I’ll have to say in almost every water feature that has a bad algae problem, it is the abundant fish load that is causing the issue. So why does an abundant fish load cause algae? When fish eat they over time, like every living creature, will have to excrete the waste (aka ammonia). This ammonia, when filtered properly, will breakdown into nitrates (aka food source). Make sense so far? This food source is then eaten by algae. From there some of the algae will be eaten by the fish and thus the cycle, the nitrogen cycle of life, begins again.

So the bottom line here is: If we have control of the food source (aka Nitrates), we have control of the algae. I have mentioned this before in the past, but it bears repeating.

Keep Fish Loads to a Minimum:
I know you love your fish and this is a touchy subject. But if you plan to have sixty 12″ koi in a 1,000 gallon pond, your going to have an algae problem and it won’t be inexpensive to get a hold of. When calculating your fish load think of it in pounds of fish or total inches. For example, one 6” fish can weigh as much as four 4” fish. The number of fish will affect the overall fish load, although 10 small fish may only produce the waste of one large fish. With this said, remember that your fish are growing and in many cases multiplying. Always plan for the future and be careful not to overstock your water feature.

Proper Filtration:
The size and type of your filtration system will depend on your total fish load. If your filter is not properly sized for max potential, your fish will outgrow the filter. When this happens, ammonia levels can reach to lethal levels. In most cases filters on the market are rated for ponds containing no fish or a minimal fish load. It is always best to get a filter that is rated for at least 2x the water volume of your pond.

Aquatic Plants:
Aquatic plants and algae will compete for the same food source in order to grow. I don’t know about you, but I would much rather see a few beautiful water liles then green slime. A simple rule of thumb is to have 60% plant coverage. This should consist of submerged, floating and marginal plants. Floating plants, such as Water Hyacinths & Water Lettuce, are fantastic at pulling nitrates from the water. I recommend putting a few into your waterfall filter box if you have one. Rooted plants, such as water lilies and marginal plants, create a great place for your fish to hide from predators. Please note when aquatic plants are not present, algae will take their place. See our selection of aquatic plants here.

Beneficial Natural Bacteria :
I’m sure you hear this a lot nowadays as to why you should be adding beneficial natural bacteria to your water feature. The reason is because it is another reducer of nitrates. One  product to check out for this is called the DefensePAC. It is a combination of five products that provide beneficial bacteria, trace minerals, and a fish and plant safe pond cleaner. The DefensePAC works to breakdown fish waste, leaves or other organics that accumulate in the pond. These are essential to maintain a clean, clear and healthy ecosystem. The best of all, one DefensePAC lasts up to 6 months for a 2,000 gallon water feature.

14 Responses

  1. My string algae is growing in my waterfall (10 foot fall).
    If I put the defense Pac into the pond (16ft x 20ft) will this loosen the algae and clear it up on the falls?

  2. Rosemary W. Reid,

    The biggest issue you have will when treating your pond is the outlet to the river. Your state may require you to have a permit to treat your pond.

    There is a great product to help combat duckweed. It’s call WhiteCap. In a pond your size, you really only need to purchase the 8 oz bottle. Although WhiteCap is expensive, its results are really amazing. Just note, that when treating the duckweed with WhiteCap, you are going to kill quite a few other submerged aquatic plants as well. For more information, click here for WhiteCap.

  3. natural, springfed pond, outlet to river, about 30′ X 80′, from 2 to 6′ deep. no filteration system, just an aireator, which may be too small. I have duckweed , even now in mid March, (the ice just went out this weekend) some fish, turtles, frogs, crabs, muskrat once in a while, and herons too. I enjoy everything except the duckweed, it COVERS the pond. Any way of getting rid of it?

  4. Floyd Van Weelden,

    Unfortunately, the number of fish in your pond really comes down to your filtration system and to make sure it can handle the fish load you have. In your waterfall filter box I suggest putting Water Hyacinth or Water Lettuce in there if you can. This will add quite a bit of extra filtration. Also, adding natural bacteria such as the DefensePAC will help remove a lot of the nutrients in the water as well.

  5. raywells,

    It will help, but I think the best thing to look at is your filtration system and make sure it can handle that type of fish load. To have 7 medium sizes fish in a small couple hundred gallon feature can pose a risk of algae blooms quite quickly. If you could, let me know what you have and I should be able to help you from there.

  6. Guy,

    LOL, I know how your feel. I didn’t make sense to me for a while either, but there are forms of algae that have the ability to grow in cold temperatures. See this post of algae growing in the winter. This is a post about a pond/lake, but its the same concept. Great comment.

  7. David Gillespie,

    I’ll keep sending them if you keep reading them! =)

    Great question, but the answer really comes down to fish load capacity of your filtration system. If you could let me know what type of system your have, I might be able to help your from there.

  8. Appreciate receiving your weekly Q&A. Could you give a guide for the number of fish that would not be a contribution to the String Alge problem for a 1000 gal pond as I have had the same problem as Steve. Thank you

  9. IS STEVES WATER FEATURE INSIDE. HOW CAN HE HAVE ALGAE ALREADY. I ALSO LIVE IN NY. THE AIR TEMP IS STILL VERY COLD. THE WATER TEMP IS COLD AS WELL. WHAT AM I MISSING?

    • Hi Guy
      I live in the Seattle Area. I have a large pond and a 95 foot water fall stream. I had String Algae all winter in my water fall stream. The cold air did not matter at all!!!

  10. I cleaned my filter today and I put the Oxy Defence in the pond and waited 20 minutes and turned the filter back on. I am not kidding I already saw stuff coming off the bottom of the pond. This stuff is amazing! Thank you Pond Guy. Marie Layne

  11. i have a small 200gal with 7 med sized goldfish this year is the frist time alge has devolped.will the DefensePac clear up my problem

  12. This same algae is producing Bio Diesel ?

  13. What do you advise for number of fish in a pond? I have about 11 fish (3 are koi) in a pond that is about 8 by ten and about 18 inches deep. I do have rocks and plants around it and waterlilies. What do advise for planting in the filter above my waterfall? I have filter at the one end where the pump is and then the water goes and over the waterfall. The waterfall filters are three pads and they are about 18 x 24. I have some algae cleaner coming from you and a heron!

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