Dozens of freshwater fish eat algae, but the most effective species for aquarium cleanup include otocinclus catfish, bristlenose plecos, Siamese algae eaters, mollies, and Florida flagfish. Each targets different types of algae, so the best choice depends on your tank size, the kind of algae you’re dealing with, and what other fish you keep.
Small Tanks: Otocinclus Catfish
Otocinclus catfish (often called “otos”) are the go-to algae eater for tanks under 20 gallons. They stay around 2 inches long, and their small, slender bodies let them squeeze into tight spots that larger fish can’t reach. Their sucker-shaped mouths are built for scraping diatom algae, the brownish film that coats glass, rocks, and plant leaves in newer tanks. They’re gentle on live plants, eating algae off the leaves without damaging the plant itself.
Otos are shy, peaceful fish that do well in community setups with tetras, rasboras, and other small species. They’re happiest in groups of at least three or four. One important note: otos are such dedicated algae grazers that they can actually run out of food in a very clean tank. If your glass stays spotless, supplement their diet with algae wafers or blanched vegetables like zucchini.
The Best Pick for Black Beard Algae
If your tank has black beard algae (also called brush algae), the Siamese algae eater is one of the only fish that will reliably consume it. These fish grow to about 6 inches and need a tank of 30 gallons or more. Their downturned mouths are well suited for eating hair algae, black beard algae, and leftover food scraps. Hobbyists report that a single Siamese algae eater can clear black beard algae from a tank in a couple of weeks.
Be careful not to confuse them with Chinese algae eaters, which look similar but behave very differently. The true Siamese algae eater has a silver body with a clean black horizontal stripe running from head to tail and tops out at 4 to 6 inches. Chinese algae eaters grow much larger (up to 10 inches), become aggressive as they age, and actually lose interest in algae over time. Older Chinese algae eaters sometimes shift to eating the scales and fins of tankmates. Siamese algae eaters stay peaceful and keep eating algae throughout their lives.
Siamese algae eaters can be slightly territorial with their own kind, so keep either one or a group of at least three to spread out any squabbling.
Bristlenose Plecos: A Manageable Alternative
Plecos are probably the most famous algae eaters in the hobby, but common plecos grow to 18 to 24 inches and need enormous tanks. Bristlenose plecos are a much better option for most people. They max out at 4 to 5 inches and do well in tanks of 25 gallons or larger. Their sucker mouths are made for scraping algae off glass, rocks, and driftwood. They also vacuum up uneaten food.
Bristlenose plecos are peaceful and compatible with most community fish, including tetras, danios, rasboras, and guppies. Like otos, they benefit from supplemental feeding with algae wafers and fresh vegetables, especially in tanks that don’t produce much natural algae growth.
Livebearers That Graze All Day
Mollies are an underrated algae eater. Their flat, grasping jaws are built for constant picking at algae on plants, rocks, and glass. They’ll graze throughout the day, and because they’re livebearers with high metabolisms, they rarely stop eating. Mollies are adaptable fish that thrive in both freshwater and slightly brackish setups, and they’re peaceful enough for most community tanks.
Florida flagfish are another strong option, especially for hair algae and black beard algae. At just 2.5 inches, they’re small enough for moderately sized tanks and have a reputation for ripping out fuzzy algae types that other fish ignore. One thing to keep in mind: flagfish prefer cooler, unheated water, so they aren’t compatible with tropical species like otos or Siamese algae eaters that need warmer temperatures.
Algae Eaters for Specific Problems
Different algae require different eaters. Here’s a quick breakdown:
- Brown diatom film on glass: Otocinclus catfish and hillstream loaches. Hillstream loaches have strong gripping abilities that let them cling to vertical glass like tiny window washers.
- Hair algae and thread algae: Siamese algae eaters, Florida flagfish, and rosy barbs. Rosy barbs are particularly effective at pulling filament algae off plants without damaging the foliage, and they’ll even eat duckweed.
- Black beard algae: Siamese algae eaters are the most reliable option. Florida flagfish will also eat it.
- General surface algae on plants and rocks: Mollies, bristlenose plecos, and nerite snails.
Not Just Fish: Shrimp and Snails
Amano shrimp are some of the most effective algae cleaners available. They tirelessly pick at algae on plants, hardscape, and substrate, and many planted tank hobbyists consider them essential. Cherry shrimp and crystal shrimp also eat algae, though they’re not quite as efficient as Amanos. Keep shrimp away from large fish that might eat them.
Nerite snails, especially the smaller horned nerite varieties, are excellent for nano tanks. They’ll slowly but steadily clean algae off glass and rocks without reproducing in freshwater (their eggs need brackish water to hatch), so you won’t end up with a snail population explosion.
Why Algae Eaters Alone Aren’t Enough
No fish or invertebrate will eliminate an algae problem caused by too much light or excess nutrients. Algae eaters are best thought of as maintenance crew, not a cure. If you’re seeing heavy algae growth, reducing your lighting period, cutting back on feeding, and staying on top of water changes will do more than any fish can. Algae eaters work best when they’re keeping a manageable amount of algae in check, not trying to outpace an algae bloom.
It’s also worth knowing that most algae-eating fish need supplemental food. A tank that’s too clean can actually starve them. Algae wafers dropped in after lights-out, plus occasional slices of blanched zucchini or kale, will keep your algae eaters healthy between natural grazing sessions.
Saltwater Tanks: Fewer Guarantees
In marine aquariums, the lawnmower blenny is often recommended for hair algae, but its reputation is better than its track record. These blennies reliably eat film algae off rocks and glass, but most hobbyists find they won’t touch long strands of green hair algae. Almost no cleanup fish in saltwater will take on long, stringy hair algae. They graze and keep surfaces mowed short, but they won’t clear an established hair algae outbreak. For marine tanks, a combination of herbivorous snails, hermit crabs, and careful nutrient management tends to be more effective than relying on a single fish species.