FAQ: Algae

contributed by George Booth

The following descriptions and control techniques are for common types of algae found in freshwater aquaria.


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There are two categories of algae of concern to aquarists: "good" and "bad". Good algae is present in small quantities, is indicative of good water quality and is easily kept in check by algae eating fish or simple removal during routine maintenance. This algae is a natural consequence of having a container of water with nutrients and a light source.

Bad algae is either an indicator of bad water quality or is a type of algae that tends to overtake the tank and ruin the aesthetics the aquarist is trying to achieve. The label of "bad" is entirely subjective. For example, one type of green, hair-like algae is considered a plague by some American aquarists, yet is cultivated by European aquarists as a valuable addition to most tanks, serving as a dietary supplement for the fish.

Algae Types

Blue-green, slime or smear algae

Grows rapidly in blue-green, slimy sheets. Spreads rapidly over almost everything and usually indicates poor water quality. However, blue-green algae can fix nitrogen and may be seen in aquariums with extremely low nitrates. Sometimes seen in small quantities between the substrate and aquarium sides. Will smother and kill plants.

This is actually cyanobacteria. It can be physically removed, but this is not a viable long term solution as the aquarium conditions are still favorable for it and it will return quickly. Treatment with 200 mg of erythromycin phosphate per 10 gallons of water will usually eliminate blue-green algae but some experts feel it may also have adverse effects on the biological filter bed. If erythromycin is used for treatment, ammonia and nitrite levels should be carefully monitored.

Brown algae

Forms in soft brown clumpy patches. In the freshwater aquarium, these are usually diatoms. Usually indicates a lack of light or an excess of silicates. Increased light levels will usually make it disappear. Easily removed by wiping the glass or siphon vacuuming the affected area.

Green water

Green unicellular algae will sometimes reproduce so rapidly that the water will turn green. This is commonly called an "algae bloom" and is usually caused by too much light like direct sunlight.

An algae bloom can be removed by filtering with micron cartridges or diatom filters. UV sterilizers can prevent the bloom in the first place. Green water is very useful in the raising of daphnia and brine shrimp.

Film algae

Grows on the aquarium glass and forms a thin haze. Easily removed by wiping the glass. Considered normal with the higher light levels needed for good plant growth.

Spot algae

Grows in thin, hard, circular, bright green spots, usually on the aquarium glass but also on plants under high light conditions. Considered normal for planted tanks. Must be mechanically removed. On acrylic aquariums, use a cloth pad or a gentle scouring pad like a cosmetic "Buff-Puff" and a lot of elbow grease. On glass tanks, scraping with a razor blade is most effective.

Fuzz algae

Grows mostly on plant leaves as separate, short (2-3mm) strands. Considered normal. It might be a less "virulent" form of "beard" algae. Easily controlled with algae eaters such as black mollies, Otocinclus, Peckoltia and siamese algae eaters.

Beard algae

Grows on plant leaves and is bright green. Individual strands have a very fine texture but it grows in thick patches and looks just like a green beard. It grows up to 4 cm. It cannot be removed mechanically. This does not indicate bad water quality but grows very fast and overtakes the tank, making it a "bad" alga. Can be eliminated with Simazine (Aquarium Pharmaceuticals "Algae-Destroyer").

Hair algae

Grows in bright green clumps in the gravel, around the base of plants like Echinodorus and around mechanical objects. It has a coarser texture than "beard algae". Beard algae will ripple in the water current, hair algae tends to form matted clumps. Individual strands can get to 5 cm or more. This is easy to remove mechanically by twirling a toothbrush in it. Can be troublesome if left unchecked. This is a popular food supplement for fish among European aquarists.

Thread algae

Grows in long, thin strands up to 30 cm or more. Tends toward a dull green color (hard to tell because it is so thin). Usually indicates an excess of iron (> 0.15 ppm). Easily removed with a toothbrush like hair algae.

Staghorn algae

Looks like individual strands of hair algae but tends to grow in single branching strands like a deer antler and is grey-green. Seems to grow mostly on tank equipment near the surface. Difficult to remove mechanically. Soak affected equipment in a 25% solution of household bleach and water to remove it.

Brush algae

This grows in feathery black tufts 2-3 mm long and tends to collect on slower growing leaves like Anubias, some Echinodorus and other wide leaf plants. Also tends to collect on mechanical equipment. This is actually a red alga in the genus Audouinella (other names: Acrochaetium, Rhodochorton, Chantransia).

It cannot easily be removed mechanically. Remove and discard the affected leaves. Equipment can be soaked in a 25% bleach solution, then scrubbed to remove the dead algae. Siamese Algae Eaters (Crossocheilus siamensis) are known to eat this algae and can keep it in check. A more drastic measure is treatment with copper.

Prophylactics for Algae

Algal spores are everywhere and will always be present in an aquarium unless drastic measures are taken. For fish only tanks, a properly set up ultraviolet sterilizer will kill algal spores in the water and prevent them from gaining a toehold.

For planted tanks, this is not a good solution since the UV light will also oxidize trace elements needed by the plants and will limit the plant's growth potential. Unfortunately, conditions that are good for growing plants are also good for growing algae. Fortunately, plants will usually out-compete algae for the available nutrients. However, if there is an imbalance of nutrients, algae will opportunistically use whatever is not used by the higher order plants. Different algae will utilize different nutrients, causing sporadic outbreaks of new algae types in apparently stable tanks when a temporary imbalance occurs.

An ounce of prevention is worth a pound of cure. To avoid introducing a new algae type to a planted tank with new plants, a simple bleach dip seems to work well. Mix 1 part bleach in 19 parts water and dip the new plant in it for 2 minutes. Immediately rinse the plant in running water, then immerse it water containing a chlorine remover to neutralize any remaining bleach. This will kill the algae and only temporarily slow down a healthy plant. Plants in poor condition may succumb to this treatment, but they probably would not have lasted anyway.

Algae Eaters

The most effective control of algae in a planted aquaria is via algae eating fish. It is especially critical in the set up of a new tank to make sure algae does not get established before the plants have had a chance to establish themselves. For this reason and to help the biological filtration get established, it is recommended that some hardy algae eaters are added right away.

Black mollies

Black sailfin mollies are excellent candidates for the break-in period of a planted tank since they are cheap and easy to find. They are usually considered expendable and are removed after a month or so. It is important to NOT FEED THEM. If they are fed, they will not be quite so eager to consume algae. When they are hungry, they are eager consumers of most algae types seen during the break-in period.

Otocinclus sp.

Otocinclus are diligent algae eaters, but are best kept in schools due to their small size. One per 10 gallons is a useful rule of thumb. Various species of otos are seen in the shops at various times; most are good algae eaters but some seem to prefer the slime coat on fish to algae. Unfortunately, there seems to be no way to distinguish the "attack otos" from normal otos.

Otos seem to be very delicate fish, but this is probably due to capture and shipping abuse rather than an inherent weakness. When a fish shop gets some in, it is wise to wait a while before purchasing to account for die offs. Most people report getting a dozen and having them die over a period of a few months until just a couple are left. Those then seem to last for a long time.

``Plecostomus'' sp.

Plecostomus is the generic name for a wide range of sucker-mouth fish. Only the smaller types are useful in a planted tank, since the larger varieties tend to eat the plant right along with the algae. Two common types that are useful are the "bristle-nose plecostomus" and the "clown plecostomus" or Pekoltia. Both stay under 4" long and don't seem to cause too much plant damage. Sometimes broad-leafed plants like Amazon swords will be scraped a little too closely by the plecos, so they bear watching.

Their diet can be supplemented by blanched zucchini and bottom feeder tablets. They also appreciate a chunk of driftwood in the aquarium to satisfy their need for cellulose. See the GOOD FIRST FISH FAQ for more information on keeping suckermouth catfish.

Siamese Algae Eater

Do not confuse this fish with the Chinese Algae Eater, which is very aggressive and does not eat algae. The siamese algae eater, Crossocheilus siamensis, is a very good algae consumer and is known to eat black brush (red) algae. The only problem is that these fish are hard to find in the United States (see the RESOURCES section of the PLANT FAQ for sources and identification paper). There are several fish in this family. The most commonly seen is Epalzeorhynchos kallopterus, commonly known as the Flying Fox. The Flying Fox is the more attractive of the two. It tends to have a brownish body with a very distinct, sharp-edged black stripe with a distinct, thin gold or bronze stripe above it. These tend to be very aggressive when they are full grown and don't eat red algae (as far as one aquarium reference is concerned).

The other member is the Siamese Algae Eater. It is the same shape as the Flying Fox but tends toward a silverish body with a somewhat ragged black stripe. There may be an indistinct gold or bronze stripe above the black. These are definitely not aggressive; they are good companions for discus and small tetras.

When they are young, the differences between E. kallopterus and C. siamensis may not be very apparent, especially if you haven't seen both types together. Unfortunately, most wholesalers don't sell fish to stores by their scientific name and the common names that are used sometimes get pretty silly (like "siamese flying fox"). If you really can't tell which one the store has, buy it anyway, but be prepared to sacrifice it if it turns out to be the wrong kind (unless your fish aren't bothered by it, of course).


Farlowella are useful algae eaters although they are very sensitive to water conditions. They type known as the Royal Farlowella will get too large for a plant tank and may cause damage.

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