How to Get Rid of Algae in Your Fish Tank

Not all algae is the same, and treating the wrong type wastes time and money. Here are the six types you will actually encounter in a freshwater tank.

Green Spot Algae (GSA) shows up as hard, flat green dots on glass, slow-growing plant leaves (especially anubias), and decorations. You cannot wipe it off with a soft cloth. It needs a razor blade on glass or a nerite snail to remove it.

Green Dust Algae (GDA) is a soft green film that coats the glass evenly. It wipes off easily but comes back within days. It looks like someone smeared green paint on the inside of your tank.

Hair or String Algae grows in long, wispy green strands, usually on plants, hardscape, and filter intakes. You can pull it out by hand or twirl it around a toothbrush. It feels soft and stringy.

Black Beard Algae (BBA) is dark grey to black tufts that grow on leaf edges, driftwood, filter outlets, and equipment. It is tough, does not pull off easily, and is one of the most stubborn types to eliminate. It feels fuzzy and stiff.

Brown Diatoms are a dusty brown coating on glass, substrate, and plants. Common in new tanks (first 1-3 months) and tanks with low light. Wipes off easily and usually resolves on its own as the tank matures.

Blue-Green Algae (BGA) is technically cyanobacteria, not algae. It forms slimy sheets of blue-green or dark green material that peels off in layers and smells terrible. If your algae smells swampy, it is probably BGA.

Algae needs three things: light, water, and nutrients. You cannot remove water or nutrients from a fish tank, so the levers you can pull are light duration, light intensity, and nutrient balance.

Too much light is the most common cause. Running your light for 10-12+ hours per day or using a light that is too powerful for your tank gives algae more energy than your plants can compete with. In tanks without live plants, this is even worse because nothing is competing with the algae at all.

Nutrient imbalance matters more than total nutrient levels. High nitrate alone does not cause algae, but high nitrate combined with zero phosphate and strong light does. Plants need balanced nutrition. When one nutrient is missing, they stall, and algae fills the gap. Inconsistent fertilizer dosing is a common trigger.

Poor CO2 in planted tanks creates the same problem. Plants in high-light setups without adequate CO2 cannot photosynthesize fast enough to use the available light and nutrients. Algae, which has much lower CO2 demands, takes over.

Poor water circulation creates dead spots where nutrients accumulate and debris settles. These stagnant areas become algae hotspots. If algae consistently appears in one corner of your tank, check whether your filter output reaches that area.

New Tank Syndrome is real. Brown diatoms are almost guaranteed in the first 1-3 months of any new tank. This resolves on its own as silicates in the water are consumed. Do not panic about brown algae in a tank less than 3 months old.

Green Spot Algae on glass is cosmetic. Scrape it off with a razor blade scraper during your weekly water change and move on. If it is growing on plant leaves, that usually means low phosphate. Test your phosphate levels. If they read 0, dose potassium phosphate (part of most complete fertilizers) to bring levels to 1-2 ppm. GSA on anubias leaves is extremely common because anubias grows so slowly that the algae has weeks to colonize each leaf.

Nerite snails are the single best solution for persistent green spot algae. They actually eat the hard spots that nothing else touches. Two or three nerites in a 20 gallon tank will keep your glass nearly spotless. They cannot breed in freshwater, so you will not end up with a snail explosion.

Green Dust Algae is trickier. The counterintuitive fix is to leave it alone for 3-4 weeks. GDA has a lifecycle, and if you keep wiping it off, you spread the spores and restart the cycle. Let it grow undisturbed for 3-4 weeks, then do a massive wipe-down of all glass surfaces followed by a large water change. This usually breaks the cycle. If GDA keeps coming back, reduce your light duration by 1-2 hours.

Hair algae thrives when there is excess light and available iron. First, reduce your light period to 6 hours per day. If you are dosing liquid iron separately, stop. Most all-in-one fertilizers contain enough iron without additional supplementation.

Physical removal works well for hair algae. Twirl a toothbrush or wooden skewer in the strands. They wrap around the brush and pull right out. Do this every few days during the initial outbreak. Amano shrimp are the best biological control for hair algae. A crew of 5-10 amanos in a 20 gallon tank will noticeably reduce hair algae within 2 weeks. They eat it constantly and will even pull strands off plant leaves.

Black Beard Algae is harder to deal with. BBA usually signals fluctuating CO2 levels in planted tanks, but it also appears in non-planted tanks with poor water flow. The nuclear option is spot-treating with hydrogen peroxide. Turn off your filter, draw up 1-2 mL of 3% hydrogen peroxide in a syringe, and squirt it directly on the BBA. Leave the filter off for 5 minutes, then turn it back on. The BBA turns pink or white within 24-48 hours and dies. Fish and shrimp tolerate small amounts of peroxide, but do not exceed 1 mL per gallon of tank water total.

For BBA on removable items like driftwood or decorations, pull them out and soak them in a 1:20 bleach-to-water solution for 2-3 minutes, then rinse thoroughly and soak in dechlorinated water before returning to the tank. Siamese algae eaters are the only fish that reliably eat BBA, but they grow to 6 inches and need a 30+ gallon tank.

Brown diatoms in a new tank are normal and temporary. They feed on silicates that leach from new substrate, glass, and decorations. In most tanks, diatoms disappear entirely after 2-3 months as silicate levels drop. Wipe them off the glass during water changes, but do not stress about them. Otocinclus catfish devour diatoms. A group of 6 otos in a 20 gallon tank can clear diatoms within days. But only add otos to a tank that is at least 2-3 months old with established biofilm for them to graze on.

If brown diatoms persist past the 3 month mark, check your water source. Some tap water has high silicates. An RO (reverse osmosis) unit removes silicates but is overkill for most hobbyists.

Blue-Green Algae (cyanobacteria) requires a different approach because it is not actually algae. BGA usually appears when nitrate is very low (under 5 ppm) and water flow is poor. Counterintuitively, the fix is often to add nitrogen. Dose potassium nitrate to raise nitrate to 10-20 ppm and increase water circulation.

If BGA persists after fixing nutrients and flow, a 3-day blackout works. Turn off the light completely, cover the tank with blankets or black trash bags to block ambient light, and do not feed the fish. BGA cannot survive without light for 72 hours, but plants can. After the blackout, do a 50% water change and manually remove any remaining BGA sheets.

As a last resort, erythromycin (API E.M. Erythromycin) kills BGA within days because it is a bacterium, not an algae. Follow the package dosing instructions. It will not harm your biological filter at the recommended dose.

Not every "algae eater" actually eats algae, and none of them will solve an algae problem caused by underlying issues. Fix the root cause first, then add cleanup crew to handle what remains.

Nerite snails are the best glass cleaners in the hobby. They eat green spot algae, green film algae, diatoms, and biofilm. They cannot reproduce in freshwater, so population stays controlled. They do leave small white eggs on hardscape that never hatch, which bothers some people. Two to three per 20 gallons is plenty.

Amano shrimp are the top choice for hair algae and soft green algae. They work in groups, so get at least 5 for a 20 gallon tank. They are peaceful, hardy, and large enough that most community fish will not eat them. They do not eat BBA or green spot algae.

Otocinclus catfish eat diatoms, soft green algae, and biofilm. They are small, peaceful, and must be kept in groups of 6 or more. They are sensitive to water quality and should only go in mature tanks (3+ months old). They starve easily if there is not enough natural biofilm, so supplement with blanched zucchini or algae wafers if needed.

Bristlenose plecos are general-purpose algae eaters that handle most types except BBA. They max out at 4-5 inches and produce a lot of waste. One bristlenose per 20+ gallons is enough. Do not confuse them with common plecos, which grow to 18+ inches and have no place in a tank under 75 gallons.

Mystery snails eat soft algae and decaying plant matter but are not aggressive algae eaters. They are more cleanup crew than algae crew. Good to have but will not solve an outbreak on their own.

Chemical algae treatments are a last resort, not a first response. They treat symptoms without fixing causes, so algae comes back unless you address the underlying problem.

API AlgaeFix works on green water, hair algae, and blanket weed. Follow the dosing instructions exactly. Overdosing kills shrimp and can crash your oxygen levels as dead algae decomposes. Never use AlgaeFix in a tank with shrimp or other invertebrates. It is toxic to them even at recommended doses.

Seachem Flourish Excel (glutaraldehyde) is marketed as a liquid carbon supplement but also kills algae when spot-dosed. Use a syringe to apply 1 mL directly onto BBA or hair algae with the filter off. It is safer than hydrogen peroxide for regular use but still stresses sensitive plants like vallisneria and certain mosses.

UV sterilizers solve green water (free-floating algae) permanently. An inline or hang-on UV sterilizer kills algae cells as water passes through. A 9W UV sterilizer clears green water in a 20-40 gallon tank within 3-5 days. It does not help with algae growing on surfaces.

Never use copper-based algae treatments in tanks with shrimp or snails. Copper is lethal to invertebrates at concentrations that are safe for fish. Read the ingredient list on any treatment before adding it to your tank.

The best chemical treatment is prevention. Consistent lighting schedules, balanced fertilization, regular water changes, and a healthy plant mass outperform any bottle of algae killer.

Preventing algae is easier than fighting it. These habits keep your tank clean without constant intervention.

Run your light on a timer for 6-8 hours per day. This is the single most impactful thing you can do. Algae cannot grow in the dark. Even reducing from 10 hours to 7 hours makes a noticeable difference within a week.

Do not overfeed your fish. Uneaten food breaks down into nutrients that fuel algae. Feed only what your fish eat in 2 minutes, once or twice per day. If there is food on the substrate after 5 minutes, you are feeding too much.

Keep up with water changes. 20-25% weekly removes excess nutrients and waste before they accumulate to algae-feeding levels. Skipping water changes for a few weeks is one of the fastest ways to trigger an algae bloom.

Maintain healthy plants. A heavily planted tank with fast-growing stem plants outcompetes algae for nutrients. Floating plants like frogbit, water lettuce, or water wisteria are especially good at absorbing excess nutrients and blocking light from reaching algae below.

Clean your filter regularly but not obsessively. A clogged filter reduces flow and creates nutrient-rich dead spots. Rinse filter media in old tank water every 3-4 weeks.

Avoid direct sunlight on your tank. Even 2-3 hours of direct sun through a window can trigger algae that your regular lighting schedule never would. If your tank is near a window, close the blinds during peak sun hours or relocate the tank.