Using Seaweed to get rid of nuisance algae in your aquarium or pond


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Using Seaweed to get rid of nuisance algae in your aquarium or pond, part 1

One of the neat things about nature is that it's had a long time to figure things out. Especially under water. Here, nature has figured out how to reduce all the "nutrients" to very low values, and also how to feed every aquatic animal (as well as consume half the world's CO2, and produce half the world's oxygen), by just using sunlight. Not bad. Maybe this concept can be used to help your water pets.

Well of course it can. And it is already doing so, sort of. It's just not being used enough, or even on purpose. It's sort of the difference between a bicycle and a motorcycle, or a snack and a big dinner. Or even knowing you have a dinner in the first place. It's called photosynthesis.

You have all heard about photosynthesis; it reminds you of trees and science experiments. But how can it help? Well the basics are this: Photosynthesis takes carbon out of carbon dioxide, and uses it to build living things, and it releases oxygen in the process. You've probably also heard that all living things contain carbon; well, that's where the carbon comes from, and photosynthesis is how it got there. The living things that photosynthesis builds generally are plants (on land) and algae (seaweed and phytoplankton) in the water. Then, anything (like you) that eats these plants or seaweeds will get the carbon you need to grow. Oops, there is one more neat thing that photosynthesis does when building these living things: It uses Ammonia, Nitrite, Nitrate, Phosphate, and many metals like Copper and Iron too.

Sounds like an ideal filter, right? Removes Ammonia, Nitrite, Nitrate, Phosphate, CO2 and metals, and put oxygen into the water. Also sounds like an ecosystem, like the ocean, or lakes, or rivers. Because it is! That's how the oceans, lakes and rivers are naturally filtered!

So we will be showing you how to build your own DIY versions of these neat filters (for fresh or salt) in the coming posts.


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Using Seaweed to get rid of nuisance algae in your aquarium or pond, part 2

So how does photosynthesis "pull" the carbon it needs out of the air or water? Doesn't CO2 or other nutrients just "soak" into things by themselves? For example, doesn't CO2 just soak into trees? Meaning, if you have more trees, won't more CO2 just soak into them? No, not really.

CO2 and other things do "soak" into water, but the reason that the water does not "fill up" with CO2 is because organisms in the water "pull" and "eat" the CO2. What organisms might these be? Algae, of course. Or more specifically Photo-Auto-Trophs (photoautotrophs), which means they get their food (carbon) all by themselves (auto), without needing to eat other animals, and they do this using sunlight (photo). In the open ocean and open lakes, all this is done by free-floating algae (phyto plankton), but as you the get to shallower areas of the reefs and lake shores (and in streams and shallow rivers), it is done by benthic (attached) algae on the bottom surfaces. We will be calling all attached algae "seaweed", even if it's in freshwater lakes, because saying "lakeweed" is a bit odd.

The faster that carbon is taken out of the water by the seaweed on the bottom surfaces, the faster CO2 can continue to absorb into the water at the water's surface. This is an important idea to understand; it forms a CO2 "gradient". This idea is easier to explain by thinking about an oven: Standing far away from an oven, you might barely feel the heat, but as you move closer to the oven, your temperature rises. So even though the oven is making the same heat, the amount of heat you feel depends on how close you are to it.

With seaweed on the bottom of the reef or lake, the amount of CO2/carbon the seaweed "feels" depends on (among other things) how close the seaweed is to the surface of the water where the air is, because this is where CO2 is being absorbed into the water from the air. This is one of the reasons (besides light) why all the phytoplankton lives near the surface of the water. Seaweed however is far from the water surface, and water that is next to the seaweed (say, 1 cm away) has had so much carbon removed that there might be little carbon remaining. So by making the air more near to the seaweed, it feels and has access to more CO2/carbon. This "near-ness" of air to the seaweed is what makes things work for our filtering needs.


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Using Seaweed to get rid of nuisance algae in your aquarium or pond, part 3

Thinking back now to the oven, and to the heat concentrated inside it, it becomes clear that if that same heat were let out of the oven, your house temperature would get hot, but not near as hot as the oven was. Why? Because the oven concentrated the heat into a small area. And, if you let that same heat outside your house, it would not warm up the outside air at all, for the same reason. If someone did open up all the windows and doors and let the heat out, you would need to stand right next to the oven to get any heat at all. So even though the same process and amount of heat exists, the area that you concentrate it in makes the difference. So going back to photosynthesis, which is the filter we want to use, the amount of filtering it does is based on the concentration of certain things:

The first is chlorophyll. This is the filtering engine that pulls the nutrients out of the water, and the more of it there is (higher concentration of algae), the more nutrient can be pulled out per hour. The second thing needed in concentration (and to keep the chlorophyll alive) is:

Light. Any part of the algae that has reduced light cannot pull nutrients out as fast. This is only up to a point however: If the light gets too strong, then the photosynthesis completely stops and the algae die. So the trick is to keep a strong, even, constant level of light concentrated on all the algae at all times, with the exception of the "off" time for the algae to rest. And let's not forget about the concentration of what we want to filter:

Nutrients. This is like the heat in the house. If you keep the windows and doors shut, you will feel more heat. But if they are open, especially if cold wind is blowing through the house, you will need to be right on top of the oven to warm up. Algae, too, need to "feel" and be near the nutrients, which are stronger near air/water interfaces that supply more CO2.

Once you are warmed up by the oven, it's time to let the next person stand next to it. Then the next, etc. You can even go into high rotation of letting many new "cold" people stand by the oven for a brief instant, while the others move through the room waiting for their turn. This is how nutrients are supplied in high amounts to the algae; a new group of air/water interfaces is brought in every second, so that the algae see and feel a high concentration of nutrients right next to them, even though a few millimeters away the nutrients are low.

So let's see how we can build such a filter that let's us concentrate these things.


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My tank gets a black substance on the plants and driftwood, any suggestions on how to eliminate this would help. I am doing lots of water changes and using safe water from Tetra.
Also, My PondHero Sludge Muncher Vacuum that i purchased online last year, rapidly clears any pond of dirt and debris, from algae and string algae to sludge and dead vegetation. Summer is the ideal time to use one when most aquatic insect larvae have emerged and tadpoles have grown into young frogs and toads.urghhh
Is this a black algae or something else or are you not sure what it is?

Even though you do plenty of water changes, you need to test the new water you are using. It is quite possible unwanted nutrients are in the new water.

Are we talking about live plants here?

How you handle a tank with live plants is much different from one that contains plastic plants.