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.

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. Will smother and kill plants.

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..

Brown algae

Forms in soft brown clumpy patches. 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. 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. You will find plenty of useful hints on keeping algae under control in the book First Aquarium Secrets.

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, Octocinclus, 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. 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.

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". Hair algae tends to form matted clumps. Individual strands can get to 5 cm or more. Remove mechanically by twirling a toothbrush in it.

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. 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 are known to eat this algae and can keep it in check.