Different algae eaters consume different forms of algae. Here are 8 popular choices, with their pros and cons described.
Siamese Algae Eaters (Crossocheilus siamensis) are small when young but gets much larger in time; to a full size of more than 5 inches (14cm). When young they voraciously eat various forms of filamentous algae, including Black brush algae (BBA). However, adults tend to want to feed on fish food instead.
Mollies (Poecilia sphenops)eat filamentous algae including BBA occasionally. However, they pick at delicate aquarium plants though and poop a lot (i.e generate alot of waste). After some time, and similar with the SAE, they might develop a preference for fish food and will only feed on algae if kept hungry.
The Amano / Yamato shrimp (Caridina japonica) is popular among planted hobbyists, the Amano shrimp is named after ADA's founder and famed aquascaper Takashi Amano. They are great algae eaters, consuming various kinds of algae and will also quickly devour leftover fish food and even deceased tank mates. However, I find that they get slightly large (6cm+) and escape open top tanks easily. They will also attack delicate/unhealthy plant leaves when hungry.
Neocaridina shrimp (Red Cherry Shrimp (RCS) comes in other color variations as well) These stay much smaller compared to Amano shrimp, and seldom attempt to escape open top tanks if water parameters are favourable. However, you will need a small horde of them to see significant impact. They breed very easily and I find that they add aesthetic value to the tank. Does not have a significant impact on tough types of algae such as BBA & cladophora.
The Farlowella/Twig/Whiptail catfish (Farlowella vittata ) look exotic and grow up to 20cm in length. They are shy and like to have hiding places, so a planted tank with areas of bog wood works well. They can be sensitive to fluctuations in water parameters, so avoid change tank parameters quickly, such as doing large water changes with tap water that is significantly different.
Oto catfish/Dwarf suckermouth (Otocinclus affinis) are small in size and harmless to shrimp and other smaller fish, making them ideal for community tanks as long as there are no aggressive fish such as larger Cichlids and Angelfish. Otos are most comfortable in groups and form loose shoals when you have more than a few. They feed on soft green algae and diatoms. They are quite sensitive and should only be introduced to a cycled and matured aquarium.
Nerite snails consume spot algae, which few other types of algae eaters eat. They prefer water with some alkalinity (5dKH +) & hardness (8 dGH+), so very acidic tanks (pH <6.5) should avoid them. They will attempt to lay eggs but raising the young to adult will require brackish water. Sometimes they will burrow beneath the substrate line, which can disturb delicate carpets.
The Bristle Pleco (Ancistrus sp.) is recommended by Tom Barr as a solution to Green Dust Algae (GDA); about a 2inch sized fish per 5 gallons of tank. They also clear up Green Spot Algae (GSA)They grow slowly but eventually reach full size of more than 5inches (14cm). One can consider buying juveniles, then selling them or exchanging them off as they reach full size. They are good at cleaning smooth surfaces such as tank walls.