Algae Control On Marine Aquariums

Saltwater algae control is quite a tricky issue that can become a big deal if left unattended. Unchecked algae will spread in no time, harming your water quality and the living conditions of your marine fish. However, getting algae control on marine aquariums right is not so difficult provided you understand why your fish tank has an algae problem and take the required steps to remedy it.

Algae is particularly annoying for beginner fish keepers, who are sold large amounts of magical chemical cures for the algae in their aquarium, instead of looking at the overall water quality and tank setup. For example, if your tank is near a window and receives natural light, algae problems are just waiting to happen. Move the aquarium, or keep the window blinds closed, before you even start trying to fix the algae problem.

Marine Algae 101: Why Is There Algae On Aquariums?

Algae are microscopic plants that are present, as developed algae or in the form of spores, in almost any body of water in the planet. This means algae live in your aquarium, even if you can’t see them. They live in the gravel, plants and surfaces of your fish tank. The only way to not have algae on a tank is by not having any sort of life on it. However, algae is only a problem when it spreads out of control and becomes visible.

Algae feed mostly on phosphorus and nitrates, and they require natural light. Fish waste and uneaten food provide the nitrates, and exposure to natural light (or in some cases, long hours of artificial aquarium light) encourage the algae to bloom. But what about phosphorus? Fish food contains it, along with the sea salt that you use to mix saltwater aquarium water. Plant fertilisers often have a lot of phosphorus, and unless you are using RO water, your tap water can also have algae food on it.

High pHs are also a reason for algae growth, which becomes particularly noticeable with a marine aquarium as the ideal pH needs to be pretty high to start with.

Are Algae Dangerous To Marine Fish?

Most aquarists dislike algae because they make their aquariums look less than ideal, but uncontrolled algae growth can also be dangerous to fish and marine invertebrates. Besides not being one of the prettiest things to look at, algae can cause changes in the water chemistry of your aquarium, creating an imbalance that can end with the death of your beloved fish and corals.

Some micro-algae are even toxic to fish, which means they can cause disease and even death if left unattended. So it’s important to deal with them and keep them under control, in order to provide your marine fish a happy environment. However, not all algae are bad algae.

Macro Algae vs Micro Algae

Laurencia, a marine genus of Red Algae from Ha...
Laurencia, a marine genus of Red Algae from Hawaii. (Photo credit: Wikipedia)

Macro algae (Red Algae, or Rhodophytes) are a welcome presence into an aquarium (usually kept on the refuge) as they compete with micro algae for nutrients, produce oxygen and help maintain pH levels. Think of them as the plants of your aquarium: they will eat up excess nutrients, and produce oxygen through photosynthesis. They are a beautiful red colour, and live on top of other algae, live rock or plants. Coralline and kelp are the most common forms of red algae.

Brown Algae, or Diatom, can be recognized by their hard, brown shells. They are generally useful, producing oxygen and removing CO2 from the water, and can be eaten by fish. The only time you need to worry about this kind of algae is when they become golden-brown, which only happens if the aquarium water is too rich in nutrients. Otherwise, it is another example of good algae.

On the other hand, micro algae are bad and ugly, and can be dangerous to your fish. There is nothing good to be said about them, and your goal is to get rid of them in your aquarium. They come in a variety of colours, from brown to green or even purple, and shapes. Some of them are not even algae, they are actually bacteria!

Encouraging the growth of macro algae will reduce the presence of micro algae (as they run out of nutrients) and will help keep your aquarium healthy and happy.

Types of Marine Micro Algae To Watch For

There are many types of micro algae, some just a nuisance while others can be actually toxic for your fish. Ideally you will remove them when you see them, as part of your marine aquarium maintenance.


The dinoflagellates are a toxic, harmful unicellular algae that varies in color from light green to brown and transparent. A dinoflagellates algae bloom is known as a red tide, and it’s very dangerous to fish in your fish tank and in nature.


As the name indicates, this is a kind of bacteria and not an algae, that forms a green slimy film over surfaces. They help fix nitrogen and encourage the growth of other, more dangerous algae.

Hair Algae

If you have ever seen algae in your tank that look like hair in a variety of green colour, hair algae is the most likely species. They grow usually in rocks, and look a bit like grass. If they look like ferns, you may have some Bryopsis Algae on your aquarium. They love growing on live rock.

Bubble Algae

This kind of algae takes a bubble-like form, looking a bit like a spawn of fish eggs. If you touch one of the bubbles they will pop and the spores will colonise your aquarium pretty quickly.

Green Caulerpa Algae

This kind of algae has leaf-like structures and will grow wildly on an aquarium if the right conditions for algae growth are present. They produce spores and also send runners.

How To Get Rid Of Algae On A Marine Aquarium

Your goal, in order to keep a beautiful and healthy marine aquarium, is to encourage the growth of macro algae while limiting the amount of micro algae. This will also make the water look clear and prevent cloudy water in aquariums.

Use RO Water Or Filtered, Clean Saltwater

RO water doesn’t have any nutrients the algae can use, and is the easiest way to keep nitrates and phosphorous on check. It will also help with the pH levels. You can also get saltwater from your local fish store, but you should filter it before adding it to your aquarium.

Keep an eye on your sea salt mix as well. Some cheap ones will have phosphates and can unbalance your aquarium, leading to cloudy, algae water. Most importantly, any sea water mix needs to be free from nitrates and phosphates, and have good amounts of calcium (needed for corals) and alkalinity. Don’t skimp on the sea water, or your aquarium will require much more work.

Perform Regular Tank Maintenance

Cleaning the tank regularly is probably the most effective way to get rid of algae. Regular maintenance removes unwanted food, waste and visible algae, which makes a surprise algae bloom unlikely.

While you are doing tank maintenance, remember to do regular water tests as well. Keeping an eye on the level of nitrates, phosphates and the pH is key to identifying a problem before it actually becomes dangerous to your fish.

Keep A Refugium

While you can keep a marine aquarium without a refugium, the benefits it has for your marine ecosystem are difficult to ignore. Refugiums are the perfect home for macro algae, which means they will keep the right levels of oxygen even when the light on the show aquarium is off, will help cleanse the waters and reduce nitrate levels. This starves the algae, which means no unwanted algae blooms.

Also, microorganisms will grow on a refugium and travel upwards to the show tank, where your fish will enjoy eating them.

Protein Skimmers

English: Protein Skimmer, used to help maintai...
English: Protein Skimmer, used to help maintain a healthy tank environment for fish and coral. (Photo credit: Wikipedia)

Having a protein skimmer may sound like an unnecessary expense, but it’s the best way to remove protein and other organic waste from the water. With no organic waste, the algae don’t have food and as such it keeps the water clean.

If you don’t have a protein skimmer, you will need to do much more aquarium maintenance, and will need to be incredibly paranoid about the amounts of food you feed your fish, as unwanted food will decompose and cause algae growth.

Turn The Lights Off (Get the photoperiod right)

Keeping your aquarium lights on longer than necessary will encourage the growth of algae. More than 10 hours a day is hardly necessary, and can give algae extra resources they want in order to spread.

Algae Chemical Help

In some cases, you will need to use phosphate removal compounds to bind the excess phosphate so the algae can’t use it. Excess phosphate also causes corals to be unable to use the calcium in the water to grow, so if your corals are not calcifying properly it can be a sign of a phosphate problem.

The presence of algae sometimes makes phosphate testing kits give a false negative, so if your corals aren’t growing and you have excess algae presence it is a good idea to add some phosphate removal chemicals to your system.

Algae Biological Control

There are many algae eaters suitable for saltwater aquariums, and besides adding to the looks of your tank they can be pretty efficient at keeping algae under control. However, as with any new fish or invertebrate, do your research before you purchase as they may not be compatible with your existing aquarium population.

Another way to control algae on a tank is by introducing live rock, even if you are not keeping a reef tank. The bacteria and microorganisms on live rock will absorb nitrates and phosphates and help the biological filtering of your aquarium. If you are not keen on how they look, you can always put cured live rock on the refugium instead of on the main tank.

As you can see, algae control on marine aquariums is closely related to water quality and proper tank maintenance. There is no need to use water clarifiers or other chemical substances that will hide the algae problem in your tank but not fix it. You can get rid of algae on saltwater tanks by making sure the conditions for algae growth are not in your aquarium, and encouraging the growth of beneficial macro algae instead.

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