Why do I have Algae in my Aquarium?


Everyone knows about algae and dreads having them. Almost every second aquarium has already experienced algae.

It is impossible to have an aquarium free from algae. You can have minimal amounts of algae but you cannot get rid of them entirely. Algae spores are everywhere and their introduction to the aquarium water or aquarium plants is inevitable.

What are Algae? And why do they grow in my Aquarium?

Algae are plants. With the exception of blue-green algae, which are bacteria, algae are plants that need the same things as our aquarium plants to live:

  • Light
  • Carbon dioxide
  • Nutrients

However, algae are among the “higher plants” in their development and are therefore more flexible in their requirements.

Which are the most common algae species?

There are several thousand types of algae and the classification presented here is scientifically incorrect, but it helps in everyday aquarium life.

Green algae

Green Algae

Green algae come in a wide variety of forms: small and punctiform spots (easy to see on the panes) or long threads. Green algae can be an indicator of a high phosphate (PO4) concentration.

Brown or diatoms


One reason could be too little or too old lighting. Diatoms cover the panes and decorations with a brownish coating that is very difficult to remove mechanically. Too high a silicate value due to tap water should be avoided, as the diatoms build their “housing” out of silicate.

Beard and brush algae

Beard und Brush Algae

Short tufts or long, slightly curled threads – this is what the most dreaded algae look like.

Once they are there, you have a medium-sized problem and you have to be very precise.

Bearded algae look similar to green algae, the difference can be seen by placing the test item in alcohol:

  • Bearded algae (red algae) turn red, green algae remain green.

Brush algae particularly like to settle on the leaf edges of slowly growing aquatic plants. They like strong currents, high pH levels, and little carbon dioxide.

Blue-Green or slime algae

Blue Green Algae

Some characteristics of this “group of algae”, e.g. the lack of a cell nucleus, speaks for the assignment to the bacteria. The aquarist cannot determine that the cell nucleus is missing without a microscope, but the rate of growth can be seen with the naked eye.

Blue-green algae very quickly cover the ground, decoration and panes with a greasy, blue-green or black coating.

The aquarium smells bad. Blue-green algae are not only ugly, but also very dangerous because their metabolism excretes what are known as toxins (toxins, just like other bacteria).

What causes Algae in the aquarium?

If algae appear in abundance in an aquarium, this is in most cases a sign that something is wrong. Because the best protection against algae is a “smooth” aquarium with magnificently growing aquarium plants that compete with algae for food. If algae are a problem for you, please use the following checklist to check your tank.

Dim Lighting in the Aquarium

Diatoms appear especially when there is too little light and high silicate levels. This can be remedied by increasing the lighting.

Tests have shown that higher plants – including our aquarium plants – prefer light with long-wave, red radiation. Algae, on the other hand, prefer blue and UV light.

Anti-algae fluorescent tubes have a blocking filter that filters UV light out of the spectral range.

If the lamps are too old, the emitted light spectrum shifts and the luminosity decreases. This cannot be seen with the naked eye, but the aquatic plants respond by reducing growth, which in turn encourages algae growth.

TIP: Make a note on the lamp when you changed it.

Algae will also thrive if the lighting is left on too long. A lighting time of 10 hours should not be exceeded. After approx. 8-10 hours the aquatic plants are finished with their photosynthesis.

An experiment you can conduct:  You will have algae problems with a darkening of 1 – 2 days and a subsequent lighting time of 8 – 9 hours per day.

Dirty filters and poor filtration

If there are algae problems, you should also check your filter.

It is often the case that algae appear in filters that have not been cleaned for a long time and that have been completely smeared. But as always, the dose makes the poison: too much hygiene damages the filter and is harmful to a well-functioning aquarium.

A device suitable for reducing algae is a UV-C water clarifier. Here, the water is cleaned of algae spores, pathogens, etc. with UV radiation. The UV-C water clarifier works most effectively against green floating algae. The effect is quickly visible and a UV-C water clarifier switched on a few hours a day reduces the problem considerably, but this does not apply to blue-green algae.

Nitrates and phosphates

These two water values ​​are “the pollution of yesterday and the day before yesterday” and arise from the breakdown of organic material such as leftover food, plant parts, fish droppings, etc

Plants need phosphate and nitrogen compounds to grow, but algae are downright crazy about these substances.

If there are algae problems, you should immediately measure the nitrate and phosphate content. The recommended maximum values ​​are: Nitrate max. 30 mg / l and phosphate max.

0.4 mg / l. A regular water change (25% once a week), moderate feeding and not too many fish ensure that pollutants cannot accumulate. A denitrifying (nitrate-degrading) filter also helps against nitrate.

Biological algae control

Fast-growing aquatic plants and algae-eating animals are good helpers. Suckling catfish such as Otocinclus, Amano shrimp (Caridina multidentata), the reddish sucking barb Garra rufa or zebra racing snails are tireless helpers.

Chemical algae control

Should only be carried out to support the cause change described above and is not the only solution. The causes, not the symptoms, need to be addressed. Most anti-algae preparations must be used exactly according to the dosage instructions, as overdosing can damage fish and plants.

You should also not use anti-algae agents that contain copper in aquariums with shrimp stock. As much algae as possible should be removed before use in order to reduce the organic load after the treatment.

Frequently Asked Questions (FAQ) about Algae in the Aquarium

Are algae bad for a fish tank?

Algae are not entirely bad for your aquarium. They play several important roles in your pond or aquarium:

Just like plants, algae generate oxygen. They increase dissolved oxygen in the water which good for higher organisms like fish

How do algae grow in an aquarium?

Almost all species of algae grow when there are excess phosphates and nitrates in the water.

Rich waste from the fish and other aquarium inhabitants acts like fertilizer to your garden. A rich byproduct of the waste is nitrogen which helps algae continuously grow.

How do you get rid of algae in a fish tank?

3 Tips to easily Get Rid of Algae in an Aquarium

Can algae remover kill fish?

You should not be afraid to use Algae removers. When used properly, they are perfectly ok for your fish. Algae removers are toxic. Some fish can tolerate them in small doses until they filter out of the aquarium. However, you should first check to see if your fish has any sensitivity to any chemicals.

What naturally kills algae?

Household borax is known to effectively kill green, black and blue algae.

The instructions are very simple:

Do copper pennies prevent algae?

YES. Really! Adding a few copper coins to your aquarium will prevent algae from overgrowing. This happens because copper has biostatic properties. In general, algae will not grow in tank that is either made of copper or has copper elements in its design. It is however important to note that the copper pennies will not completely eliminate algae and bacteria growth in the tank. You will still need to go for other options of preventing and eliminating algae.

How do I stop algae growing in my fish tank?

You can stop algae from constantly growing in your aquarium tank by maintaining a healthy tank. You need to make sure that you are doing the following:

  • Make sure you make weekly water changes.
  • Make sure you regularly use phosphate, silicate and nitrate removers
  • Check your lighting. Make sure it is optimal for your tank
  • Add a small bale of barley straw into the tank
  • Add some aquatic pond snails that eat algae

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