Preventing Algae

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Green string algae

What is Algae?

Algae are basically a primitive plant or photosynthetic organism that does not have a vascular system (complex structure). It is impossible to eliminate algae from entering the aquarium; it will arrive through tap water, from the air, from fish, and from plants. 'Prevention' methods such as rinsing plants & décor and not allowing new water to enter the tank when introducing fish will not work. Complete prevention is impossible and a low level of algae growth is a healthy thing, providing an extra source of food for fish and even processing some waste products

What causes algae blooms?

When algae grows quickly and becomes unsightly, it is referred to as an algae 'bloom'. Algae basically need water, light, and a source of nutrients to thrive and it is the nutrients that are the precursor to algal blooms. Although strong lighting is often blamed, with the exception of direct sunlight, lighting is rarely a cause. In terms of nutrients, there are four main conditions which will cause algal blooms; organic matter, phosphates, nitrates, and excessive water borne minerals and nutrients.

Organic matter

Waste 'organics' such as fish waste, leftover food and tank 'mulm' all break down and produce water-born nutrients, which are ideal food for algae. Correct feeding amounts (so that no food is left over) and regular substrate cleaning with a gravel cleaner will help to keep organic matter at low levels, and improve the general health of the tank. For planted tanks, or where disturbing the gravel is undesirable, a fine substrate can be used rather than larger gravel. Fine substrates allow less water to flow through the grains, which means less oxygen, and this allows nutrients to settle and become trapped to organic particles in the substrate, away from the water where algae will grow. This method helps keep the nutrients available for plants (via the plants roots) and unavailable for algae. Ideal grain size should be no smaller than 1mm (silver sand is too small) and no bigger than 2mm (e.g. pea gravel)

Phosphates & Nitrates

These two substances are major causes of algal blooms, and are constantly produced in the aquarium, building up over long periods. Regular water changes may keep nitrates down to acceptable levels but tap water often contains high phosphates. The only way to check your levels is to use a test kit and regularly check for both phosphates (PO4-), which should be under 1mg/l, and nitrates (NO3), which should be under 50mg/l. If the levels of either of these are too high, you can use specifically designed filter medias or treatments to remove both nitrates and phosphates. The best products to use depend on your tanks filtration type, for external filters there are several filter medias which can be used. For internal filters you may need to use more specialized liquid or substrate additives, or even purchase a second small internal filter to house the media's normally used in external filters..

Excessive minerals and nutrients

Tap water contains a large amount of many different minerals and nutrients, which are ideal food for algae. This is why, along with fluctuating water conditions, algae blooms are common in new aquariums or after large water changes. Using fast growing plants and absorbing filter mediums such as activated carbon in new tanks will help to keep excess minerals and nutrients at low levels. Water changes are an essential part of maintenance but should always be kept small and never exceed 20% with the exception of emergencies.

Natural preventatives

The best method to prevent and remove algae in the aquarium is to use some natural algae-eating fish and shrimps. For small aquariums and/or with small fish, Dwarf Plecs (Peckoltia sp.), Otocinclus Catfish, and Japonica/Algae Shrimps (Japonica sp.) are best. For medium tanks, Bristlenose Catfish and Siamese Flying Foxes are suitable whilst for large tanks or with big tank mates, Common Plecs and Algae Loaches will eat algae. It should be noted that only shrimps and Siamese Flying Foxes will eat 'thread' type algae. Larger algae eaters such as bristlenose or common plecs are not recommended for planted tanks since they will damage delicate leaves.

Chemical treatments

If other methods are not working, there are a wide range of anti-algae treatments available. These are best used in conjunction with the other methods mentioned, since they are only a short-term measure. Choose an algae treatment suited to the type of algae you have, and be aware that some treatments have a limited effectiveness. If possible, try natural methods first. Most algae treatments will not work if the causes of algal blooms (e.g. high phosphates, nitrates) are still present in the aquarium


"Algae is caused by too much light"

Brighter lighting will encourage algae to grow, this is a natural part of an aquariums environment but algae blooms or problems are very rarely caused entirely by lighting. It is much more likely to be an excess of organic waste, nitrates, or phosphates which causes algae blooms. It is much better to have bright lighting and healthy growing plants (which will remove nutrients from the water) than excess nutrient waste, dim aquariums, and algae.

"Snails can be used to remove algae"

Snails do eat algae, but in freshwater aquariums you would need a lot of snails to control an algae problem, by which point you would probably have a snail problem! Snails will usually not touch thread or brush algae either.

"A 'siesta' period (switching the lights off) during the day will stop algae growing"

There is some truth to this although it does not work under 'real-world' conditions. Algae need extended periods of light to thrive whilst plants can grow with intermittent light, so a siesta period in the middle of the day is designed to remove long periods of lighting, leaving lights on for no more than four hours followed by an hour of darkness. This will only work if the period of darkness is complete darkness. Under normal conditions there is always ambient light from the surrounding room, allowing the algae to continue growth and preventing harm.

"Algae is introduced with aquatic plants"

This can be true, but then again, algae is introduced by many other methods, so it is not significant. Aquatic plants often simply provide a good surface for algae to grow on as they are higher up (near light) and in the open water where nutrients are available. Healthy, growing plants actually help to prevent algae from growing so should be kept whenever possible.


1) Test regularly for Phosphates and Nitrates
2) Try keeping some fast growing aquarium plants
3) Avoid direct sunlight
4) Avoid large water changes using tapwater
5) Use phosphate & nitrate absorbing filter media
6) Use a gravel cleaner, or use a fine substrate
7) Avoid the use of liquid fertilizers when excess algae is present
8) Introduce some algae-eating species (shrimps and dwarf plecs for tanks with small fish, bristlenoses and Siamese flying foxes for larger communities)
9) Only use algae-killer treatments as a last resort
10) Avoid overfeeding and leftover food

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