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What happens with algae in aquaponics?
What happens with algae in aquaponics?

Algae can cause many issues in aquaponics but there are ways to combat it.

Written by Halle Brake
Updated over a week ago

Algae commonly occurs in aquaponic systems. Over time, algae populations stabilize in systems, but when systems are just getting started, algae can be very frustrating, locking up valuable nutrients and causing pH values to swing wildly throughout the day.

The Issues With Algae

The algae that occur in most aquaponic systems and cause the most problems are green algae. Green algae is a type of algae that consists of single celled and multi-celled organisms with chloroplasts- just like plants. In aquaponic systems, there is typically plenty of single celled algae, and often times multi-cellular algae as well.

Suspended algae can cloud up the water in aquaponic systems, foul pipes and pumps, and cause physical problems, but the largest dangers of algae in aquaponic systems are that:

1. Algae can die off or accumulate in grow beds and consume oxygen, leading to dangerously low levels of oxygen when electricity or circulation fails. Algae also consume oxygen when it is dark, and can deplete system DO (dissolved oxygen) if too much accumulates in the system

2. Algae can cause pH swings that negatively affect system health and confuse beginning aquaponic practitioners. These swings are called diurnal pH swings.


Decomposition of algae isn't typically a problem in aerobic environments.

Oxygen consumption happens when dead algae begins to decompose. This isn’t usually an issue if it happens in grow beds, or other aerobic environments. Where it’s really an issue when it happens in the fish tanks or other aquatic environments.

Decomposition consumes a lot of oxygen, so when the decomposition of algae (or any other organic waste for that matter) happens beneath the surface of the water, often oxygen is consumed faster than it can be replenished. When this happens, all of the aerobic organisms in the system suffer, including bacteria, fish and plants.

For this reason, accumulating algae should be removed from the system as it begins to die off, or minimally, more aeration should be provided to the system as algae blooms begin to die off (either through aeration or circulation).

Algae and Dissolved Oxygen

Algae can also cause oxygen problems when it’s dark. During the day, algae essentially create their own oxygen by splitting water molecules. As they create oxygen, they’re also consuming CO2 to build sugars.

At night, when photosynthesis ceases, the algae begin to consume oxygen, but cease to produce it. This can lead to oxygen depletion in the dark.

In the middle of an algae bloom, many practitioners who are measuring dissolved oxygen (DO) can be very confused. They might notice that their fish are stressed or dying, but all system variables are OK, even DO, which they measure late in the morning or afternoon. However, if they sneak out at 2 or 3 in the morning to measure DO, they might discover the source of all of those stress symptoms- DO depletion due to too much algae consuming too much oxygen in the dark.

Algae and pH

Algae cause pH swings because they consume CO2 during the day when the sun is high and photosynthesis is at it’s peak. CO2 is a weak acid in your system solution, so as the algae remove it from the water, the water pH will rise, oftentimes as much as a point or two! This can be very confusing for the beginning grower as pH may be low in the morning and shoot up in the late morning and afternoon, only to drop again late in the evening. If you don’t understand that algae in your system is causing the problem, it seems like your system chemistry is going crazy!

Combating Algae in Aquaponics

Before you go all Bruce Lee on the algae in your system, make sure that you're taking measures to prevent the growth of algae; make sure that your temperature range is appropriate to the crops and fish you have, and measure phosphorous to make sure that it's not too high.

To go on the offensive and deal with algae, the two easiest methods are mechanical filtration and shading.

I also use a special trick of using humic acid additions to shade the water.

1) Shading:

Shading is the easiest method to reduce algae in your systems. Green algae needs light to grow and reproduce, so to get rid of them easily, simply shade your fish tanks with a dark colored tarp or piece of plastic. Many growers will also paint the white (and often transparent) plastic used to hold system water black (on barrels and IBCs) or do a layer of black paint and then white (to prevent the water from heating up), to prevent algae accumulation on the inside of the plastic.

Similarly, if you struggle with algae accumulation on the surface of your gravel, shade the surface by adding enough rock or gravel that the surface of the water is covered. Shading denies green algae the light it needs to survive, and will dramatically reduce a system’s problems with algae.

2) Mechanical Filtration:

In many systems mechanical filtration plays a big role in algae removal. Oftentimes this equipment is very expensive, but it can sometimes be made inexpensively. This type of equipment includes filters, screens, vortex and centrifugal settlement equipment, settlement tanks and other mechanical means of removing algae from the solution.

In our greenhouses we use ZipGrow Towers, which act as a massive mechanical filter, physically capturing and removing algae from the system. This is another great method for removing algae, and many systems employ physical removal whether they realize it or not. If you have a grow bed on your system, it functions as a mechanical filter, straining algae out of solution.

3) Other Techniques [a trade secret]:

There are many other techniques, including the use of Ultra Violet clarifiers, barley straw, etc., but one of my favorite techniques is the use of humic acid to “darken” your system water.

In very shallow water, adding humic acid to a system can actually stimulate algal growth, but in deeper waters, the use of humic acid darkens the water and shades out the depths of the tank. Humic acid is a great addition to your system whether or not you have issues with algae. It’s great for your fish and your plants and can help chelate a number of important plant nutrients. The shading effect it exacts on algae in deep systems is really just a bonus.

Some of the other techniques out there are worth exploring, but are oftentimes too messy and expensive to justify.

The Ultimate Algae Treatment:

In the end, the best treatment is a combination of shading and filtration combined with patience. Algal blooms are an important part of any system establishment.

"When you build a system, you’re really constructing an ecology, and with that ecology comes a break-in period, where new organisms are colonizing the system, fighting for control and finally reaching a steady state- establishing a stable population."

Algae is great at colonizing, and will always be an important part of nutrient cycling in any system, but it seldom causes serious problems in the long run. Give it enough time, and it will eventually die back and find a balance.

So be patient, and see what happens. The hardest part of establishing any aquaponic system is waiting! Shade, filter and wait, and your algae problems will eventually disappear.

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