What algae eaters are for sale?
Most omnivorous fishes will eat some algae, with cichlids and livebearers being particularly happy to graze on algae if nothing else is on offer. However, only a few species of fish and invertebrate are sold primarily as algae eaters. Catfish from the family Loricariidae, known colloquially as "plecs", are perhaps the best-known of these.
Otocinclus -- Sometimes called "ottos". Typically 3-4 cm long, usually black or mottled grey. Being small, they are often recommended for small aquaria; however, they are very delicate, particularly when newly imported, and will not survive long in immature aquaria. They need stable water conditions, lots of oyxgen, and a mixed diet including algae, soft vegetables (such as slices of cucumber), and bloodworms.
Peckoltia -- Occasionally sold as "clown plecs" but more often under their Latin name. From 5 to 10 cm long depending on the species. Some species are nicely patterned while others are rather drab. Not particularly delicate, once they are feeding, so for inexperienced aquarists these are a better choice for small tanks than Otocinclus. Feeds primarily on green algae, but will also take soft vegetables.
Ancistrus -- The popular bristlenose plec. Grows to around 8-10 cm. Widely sold, inexpensive, hardy, and long-lived (7 years no uncommon) these are very good fish for beginners. Will eat algae as well as general aquarium foods such as bloodworms, mussel, catfish pellets, etc. The most common species, Ancistrus dolichopterus is not particularly attractive, but the bristles on the males certainly make them interesting to look at.
Hypostomus, Liposarcus, Pterygoblichthys -- These are the common plecs. While many aquarium books will cite Hypostomus plecostomus as the common plec, this species is hardly ever imported any more, and the species you are most likely to see is in fact Liposarcus pardalis. Exact identification of the fish doesn't matter. All these are big catfish (30+ cm) that tend to be territorial towards one another but harmless towards other fish. They are hardy, and as juveniles will eat algae. As they mature, they become less effective, if only because they cannot perch on plant leaves or crawl into small spaces to get at the algae there. Besides algae, they also need soft vegetables and some meaty foods such as prawns, bloodworms, or mussels.
There are lots of other loricariid catfish sold, including species of Chaetostoma ("bulldog plecs"), Farlowella ("twig catfish"), and Panaque ("royal plecs"), but these fish tend to be more demanding and/or less effective algae eaters.
Hillstream loaches, family Homalopteridae, are an obscure group related to the carps and loaches, despite a superficial resemblance to the plecs. They are algae eaters, and most are rather small, around 5 cm in length, so could make good aquarium fish. However, they are delicate, and need water that is subtropical (i.e., 18-22 C), well oxygenated, and completely free of nitrite and ammonium. In other words, these are fishes for a mature, specialised tank rather than a beginner's community aquarium.
Several genera are traded, including Homaloptera, Gastromyzon, and Pseudogastromyzon. These go by a variety of names, including "butterfly loaches", "Hong Kong Plecos", and "Borneo suckerfish". They have a relatively consistent appearance. The pectoral and pelvic fins are expanded to produce a wide sucker with which they stick onto rocks or glass. Many species are attractively patterned.
Sucking Loaches and Garra spp.
The sucking loach, or Chinese algae eater, Gyrinicheilus aymoneri is a fish best avoided. While cheap, hardy, and not unattractive, it is a giant fish by aquarium standards (easily reaching 20-30 cm) with a mean attitude to boot. While possibly useful alongside large cichlids or catfish, it simply cannot be housed in a regular community tank.
Garra spp. could easily be mistaken for small sucking loaches. There are variety of species, some tropical, other subtropical, but all need clean, well oxygenated water. Garra taeniata, for example, is a true tropical while Garra pingu needs cool water. Check with your retailer before purchasing, and if in doubt, get the Latin name and do a Google search or visit Fishbase to find out if the species on sale is right for your tank. Garra spp. typically grow to around 10-12 cm and are quite peaceful. They are good algae eaters.
Sharks and Foxes
These are a mixed bag. The Siamese Algae Eater, Crossocheilus siamensis (sometimes Epalzeorhynchus siamensis) is a first-rate algae eater that will work well in most aquaria. It is territorial towards its own kind unless kept in large groups, but is otherwise fairly peaceful. The Flying Fox, Epalzeorhynchus kalopterus is almost as good but tends to be more aggressive and should be kept singly and with tankmates able to swim away from trouble. Red-tailed and ruby sharks, on the other hand, are indifferent algae eaters. While they certainly make good pets for other reasons, they shouldn't be bought as algae eaters.
Mollies and other fish
Often overlooked, mollies can make excellent algae eaters in the right tank. They will peck away at algae on fine leaves that catfish and the other, more benthic fish leave behind. However, they do need hard, alkaline water, ideally with a little salt added. While livebearers and some other kinds of freshwater fishes do well in slightly salty water, most do not. Mollies are therefore best kept only in tanks where adding salt is possible.
Other fishes that will also eat a significant amount of algae include: platies, Florida flagfish (Jordanella floridae), barbs, scats, violet gobies (Gobiodes spp.), and the herbivorous "mbuna" cichlids. While not commonly kept as algae eaters per se these fish do appreciate some algae in their diet.
Shrimps have been in vogue as algae eaters for some time now, with at least one species being known as Amano shrimps after the aquarist who first popularised them. However, while shrimps can make good algae eaters, they are not without problems. They are small, for a start, typically around 1.5 to 3 cm in length, and substantially larger fish will simply view them as live food. They are also rather delicate, and need excellent water quality. They do not do well in very soft, acid water. Finally, being rather small, you need a lot of them to see much impact in large aquaria.
Snails are the other invertebrate sold as algae eaters. Most will eat a great deal of algae, particular from the glass, but some species will also eat plants as well. Apple snails and Colombian ramshorns are notorious for this. Nerite snails (e.g., olive snails) are more trustworthy, but they need brackish, not freshwater, to do well long-term. Malayan livebearer snails are excellent for cleaning the substrate, and will eat some algae, but because they breed very quickly, many aquarists do not like having them in their aquaria. Nonetheless, if you want a snail that is adaptable and does no damage to living plants or fish, then the Malayan livebearer is probably your best bet.
Will these fish beat algae?
In a word, no.
What! But they're algae eaters, aren't they?
Algae is a characteristic of unbalanced aquaria. Over time, the waste produced by the fishes in the tank are turned into nitrate and phosphate by the biological filter. Nitrate and phosphate are fertilisers, and the more of these chemicals in the water, the easier it is for the algae to 'bloom'. Therefore, the more fish you add to a tank, the more nitrate and phosphate, and thus the quicker the algae can grow. It doesn't matter if the fishes are algae eaters or not, simply putting them in the tank makes life easier for the algae.
There are two ways to solve this problem. The first is to carry out frequent water changes and thus keep the nitrate and phosphate levels as low as possible.
The other (and easier) approach is to use plants.
Using plants to stop algae
George has written a great introduction to this topic here:
Algae in the Planted Aquarium
My own 180 litre aquarium is quite heavily stocked (including, a 15 cm Panaque, 9 glassfish, 17 cardinals, 6 halfbeaks, 3 dwarf upside-down cats, a pufferfish, etc.) and at times the nitrates go as high as 100 mg/l. But I have hardly any algae, and the only place hair algae appears is on dead leaves floating at the top. The glass is cleaned perhaps once every 2 months, and there's not a trace of the blue-green algae you might expect in a tank with high nitrates.
What's the secret? Lots of plants. I'm cutting back Cabomba and scooping off Salvinia every week, sometimes twice a week. These rapidly growing plants simply put the algae out of business. I should note my system isn't complex or expensive: two 30 W Triton tubes, reflectors, and a bit of pond soil in the substrate are the only concessions to plant growth. There's no laterite, or CO2 fertilisation, or high-output lights.
Adding fish, any fish, improves the conditions in the tank for algae, so algae eaters are counterproductive if used as your sole method of algae control. Instead, you need to balance the tank by adding live plants. These will stop the algae growing in the first place, resulting in a tank that needs much less looking after and is a healthier place for your fishes to live.- Tropic Fish