Are algae in fish tanks good or bad?

Here, we will respond to the question “Are algae in fish tanks good or bad?”. We will talk about the algae biology and structures, types of algae, how to avoid and remove the harmful algae in your fish tank.

Are algae in fish tanks good or bad?

Algae in a fish tank is good. In a fish tank, freshwater or saltwater, aiming for a biological equilibrium is fundamental to the algae presence. Algae purify the water and support the tank oxygenation. They also transform inorganic minerals into organic compounds, which are then transformed into natural fish food. 

Who are the algae?

Algae are organisms that inhabit humid and aquatic environments. They can be uni or pluricellular and generally have cellulose in your cell composition. They reproduce in many ways, making it possible for rapid and efficient propagation when in favourable conditions. 

Algae are also very efficient biological indicators. They have chloroplasts, which allows them to photosynthesize. This chloroplast presence provides colours to the algae according to the pigment they own besides chlorophyll a.

The photosynthesis process transforms inorganic substances into organic compounds. Thus, algae are classified as primary food sources. The animals that feed on algae are called primary consumers. These animals can be fish, crustaceans, molluscs, insects and others. Algae also provide minerals and vitamins for their consumers.

Like any other live organism, algae require minimum conditions for their reproduction and development. This means pH, lighting, water change for nutrient availability are important and classified as favourable conditions to algae success.

Types of algae?

Although algae are not directly harmful to fish, when overgrowing, they give a bad aesthetic to your fish tank and might compete with your plants for nutrients. Algae can also be a good water quality indicator. Some types of algae only grow under specific conditions that might be harmful to your fish health.

As there are many types of algae, to avoid overgrowth or remove them from your tank, we must first learn how to identify their different types:

Green algae

Green algae, as the name already says, are easily identifiable by their green colour. They form layers resembling a hairy rug and might show some filamentous structures. 

The green algae presence can sometimes make your tank look more natural and can also feed some of your fish and other animals. However, in an overgrowth scenario, they may turn the tank water green.

In normal quantities, they are not harmful to fish and plants. However, they might become too dense and numerous that may affect your plant growth due to nutrient competition. Also, when overgrowing, green algae can indicate that your tank water is unbalanced, which could cause issues for your fish and other organisms.

To avoid green algae growth you can reduce light periods, test your water for nitrates and CO2 levels, and if necessary, correct them, and remove manually or vacuum the green algae. Also, overfeeding could improve green algae growth.

Blue-green algae

Blue-green algae develop as a blue-greenish viscous bed over rocks, plants and décor. Sometimes they can look very similar to green algae.

The presence of blue-green algae is a huge alert signal. They only grow in nitrites, nitrates, and phosphate at high levels. They can cause a musky odour and form a generous foam cover. Also, Blue-green algae are very fast-growing and spread quickly.

To avoid and remove this type of algae:

–       Stronger water flow;

–       Filter cleaning and functionality check-up;

–       Stir the substrate to put algae in suspension to be filtered;

–       Vacuum your tank.

Red algae

Red algae that can overgrow in fish tanks typically grow in a beard-like shape. They form very thick hairy clusters over plants, rocks, and décors. This group is the toughest to remove from your tank.

To fight red algae, checking CO2 levels, improving the water circulations and increasing light would be efficient practices.

Brown algae

Brown algae normally look like dust covering your tank structures. They usually occur in high levels of silicates and phosphates waters. Thus, it is very common in newly set up fish tanks. As they rely on an excess of phosphates and silicates, once these substances are consumed, they just drop the growth rate.

However, they can overgrow in some conditions in older tanks. They grow in high levels of nitrates and excess feed and fish waste. These conditions can be harmful to your fish if not treated. 

To avoid brown algae bloom, you should:

–       Change the water routinely;

–       Control tank lighting;

–       Improve filtration system;

–       Avoid overfeeding.

Filamentous algae

This category refers to many types of algae that form wet hair-like structures. Some examples of filamentous algae are staghorn algae, string algae and thread algae. 

Their overgrowth is usually associated with poor water conditions. Dirty waters with feed and fish waste are more susceptible to this algae growth. Sometimes they can be related to iron levels.

To fight filamentous algae you would need to:

–       Avoid overfeeding;

–       Stir the substrate up and routinely vacuum the tank;

–       Check your filter functions and maintain it clean;

–       Adjust CO2 levels;

–       Routinely change the tank water.

How to avoid algae outbreaks?

In general terms, you can naturally avoid an algae outbreak using some general practices to keep your tank water and your fish healthy.

Have an efficient filter

Having a good strong filter can help you to remove iron, nitrates, wastes, and other elements from your tank. Thus, it will assist you in keeping your tank free of an algae outbreak.

UV filters and microfilters can improve even more this support to fight algae blooms. In case you opt for using tap water in your tank, you should also have a high potency filter due to the presence of excessive contaminants. 

Always clean your water

To keep a fish tank healthy, you need to clean your tank every three or four weeks. However, if you have fish that naturally create more waste, you should make water changes more frequently. ¼ of the tank water is the ideal to be changed during cleaning.

Dirty tanks favourable algae overgrowth. However, some algae types only grow in conditions that are very harmful to fish. Thus, if some of these algae are growing, you should immediately adjust your water parameters to maintain your fish’s health. Otherwise, if not adjusted, these conditions may reduce your fish life expectancy.

Control lightning

You should research your fish species and adapt this factor according to their biological needs. Too much light will typically favour algae growth. Sunlight favours overgrowing more than other sources of light, thus keeping your tank with artificial controlled light would make it possible to control algae outbreaks.

Live plants

Keeping live plants in your tank can also be difficult and avoid algae overgrowth. Plants consume the same nutrients algae may need to grow. Thus, by raising plants, conditions become adverse for algae growth.

Algae eaters

In addition to creating nutrient competition with live plants, adding algae eater organisms will also help you with algae control and overgrowth avoidance.

However there are many species of algae eater fish, you should always consider the species you already own before adding new species. This is to avoid aggressive behaviours and hierarchy dominance competition. Nice algae eater fish you should consider are:

–       Catfish (Otocinclus and Twig);

–       Siamese and Chinese algae eaters;

–       Bristlenose plecos;

–       Live-bearers (Mollies and Swordfish);

–       Butterfly Splitfins.

There are also other organisms different from fish that may help you in keeping algae controlled. Some of them are:

–       Snails (Mystery, Nerite and Malaysian trumpet);

–       Shrimps (Cherry and Amano).

Do not overfeed fish

As we stated above, overfeeding your fish can create an environment rich in suspended organic material. Because excessive food dissolves and becomes suspended and fish may generate more biological waste.

To detect if you are throwing too much food you can:

–       Offer the food to your fish;

–       Wait 5 minutes;

–       If there still is food in the tank, you are overfeeding them;

–       In case you have overfed them, use a scoop to remove the food waste from the water.


In this blog post, we answered the question “Are algae in fish tanks good or bad?”. We learnt about the algae biology and types, and also how to avoid overgrowth and remove algae from your fish tank.

Any thoughts or doubts? Feel free to leave us a comment below!

Frequently Asked Questions (FAQs): Are algae in fish tanks good or bad?

How do I eliminate green algae from my fish tank?

An ultraviolet filter can help you with this mission. It works by killing green algae that are in suspension (green water) and also ends up sterilizing the water in the fresh or saltwater tank, which would assist you in avoiding fish parasite infestation. Also, check pH levels and brush off any fixed algal structure from rocks or the tank glass.

What eats algae in a fish tank?

Many organisms eat algae in fish tanks. Between them are shrimps, snails and even fish. You should always focus on the species you already raise in your tank. Because it is necessary to consider competition and aggressive behaviours between territorial fish species.

Why is my fish tank water green?

Green water is caused by suspended green algae. Some conditions favour green algae bloom. This algae species does not grow fixed over substrate or tank structures. They reproduce very rapidly and this population growth is responsible for turning the tank water green.

How fast do algae grow in a fish tank? 

Algae outbreaks can happen very quickly. Some types of species, such as the blue-green algae and green water algae, grow very rapidly. Green water algae can reach up to a billion in a short period.

Do LED aquarium lights cause algae growth? 

LED lights do not favour algae growth. However, having a maladjusted light period may favour this overgrowth. It is not the type of light that determines if algae are overgrowing or not, but the light availability and its intensity.

What do algae look like in a fish tank? 

Algae typically look like hairy furry mats. They can vary in colour and shape. From blue-greenish to black-reddish. Some of them look like small furry balls and soft films over tanks structures, plants, filters and glass.


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