How to beat aquarium algae: ID and fix every common type

Almost every algae product on the shelf is sold as a cure. Pour it in, algae dies, problem solved. Except it isn't, because the algae was only ever a sign that your tank had spare light and spare nutrients sitting around for something to eat. Kill this batch and the conditions that fed it are still there, so the next batch moves in.

The actual levers are nutrients, light, and flow. Nutrients mean nitrate and phosphate, which come from food and waste. Light means how strong your lighting is and how many hours a day it runs. Flow means whether water moves everywhere or sits dead in corners. Get those three in balance and algae starves out on its own. That's the whole game.

This is also why we'll often talk you out of buying something. If a $15 bottle would actually fix it for good, we'd sell you the bottle. Most of the time the fix is free: a shorter light timer, less food, a water change, a moved powerhead. We'll tell you straight which is which.

If your tank is a few weeks to a couple months old and a brown, dusty film is coating the glass, sand, and decor, that's diatoms. It wipes off easily and looks like someone dusted the tank with cocoa powder. Nearly every brand-new tank, fresh or salt, goes through it.

Diatoms feed on silicates that leach out of new sand, rock, and even tap water while the tank establishes. The good news is you mostly don't have to do anything. As the silicates get used up and the tank's biology matures, diatoms fade on their own, usually within a couple months. Wipe the glass, do your normal water changes, and wait it out.

If you want to speed it along, a cleanup crew helps. In freshwater, otocinclus catfish and nerite snails graze diatoms hard. In saltwater, trochus and astrea snails do the same. If diatoms hang on well past the new-tank phase, your source water may be high in silicate, which is one reason reef keepers run RODI water. Bring a sample in and we'll test it.

Green spot algae shows up as small, hard green dots stuck to the glass and on slow-growing plant leaves like anubias. It's tough, doesn't wipe off with a sponge, and usually needs a razor blade (an acrylic-safe scraper on acrylic tanks) to clear the glass. A little is normal. A lot means your light runs too long or too strong. In a planted tank it can also pair with low phosphate, which is a balance issue rather than a reason to strip every nutrient out.

Green water is a different beast and dramatic with it: the whole tank turns to pea soup overnight, a free-floating bloom you can't wipe away because it's suspended in the water column. It's almost always too much light (including direct sun on the tank) plus excess nutrients. A multi-day blackout and big water changes help, but the reliable fix is a UV sterilizer. The unit zaps the floating algae as water passes through, and a green-water tank usually clears in a few days.

For both, pull your photoperiod back. Most display tanks do fine on roughly 6 to 8 hours of light a day on a timer. Move the tank out of direct sunlight if you can, and stay on top of water changes. Just don't starve a planted tank of all nutrients chasing green spot, because that causes its own problems. If you're unsure, bring your numbers in and we'll match a plan to your setup.

Long green strands, soft and stringy, that drape off rocks, plants, and equipment and feel like wet hair when you grab them. This is the one that takes over a tank fast, and it's the clearest signal there is that you're running too many nutrients. It's most common in tanks that get fed heavily, get a lot of light, and don't get enough water changes.

First move is manual removal, and it's oddly satisfying. Twist the strands onto a toothbrush or an algae scrubber, almost like winding spaghetti onto a fork, and lift them out. Get as much physical mass out as you can before you do anything else, because every strand you remove is nutrients you remove with it.

Then close the tap. Cut feeding back, since most fish are fine fed once a day and a lot of hair algae traces straight back to overfeeding. Trim the light timer down. Bump up your water changes for a few weeks to pull nitrate and phosphate down. Healthy fast-growing plants out-compete hair algae for the same nutrients, so a tank that's struggling to grow plants will often struggle with algae too. Amano shrimp and some snails graze it, but treat them as a supplement to fixing the nutrients, not a substitute.

Dark gray to black fuzzy tufts, almost like tiny brushes, that grow on the edges of plant leaves, on driftwood, on equipment, and especially around the outflow of your filter. It's technically a red algae, and it's stubborn. It doesn't scrub off easily and snails mostly ignore it. It tends to show up where flow is moderate and, in a planted tank, where CO2 is low or swinging.

The targeted fix is spot-treatment. Turn off your pumps, draw up some liquid carbon (a glutaraldehyde-based product like Seachem Excel) or hydrogen peroxide into a syringe, and squirt it directly onto the BBA. It usually turns red or pink over the next day or two as it dies, then the cleanup crew and water changes carry it off. Follow the product label for dosing, especially around shrimp or sensitive plants like vallisneria, which don't love liquid carbon. If you'd rather pull a piece of affected driftwood or hardscape and treat it in a bucket, that works too.

The longer fix in a planted setup is stable, adequate CO2 and consistent flow, because BBA thrives on fluctuating CO2 and dead spots. In a non-planted tank, treat the affected pieces and improve flow so water isn't stalling in the same corners. As always, keep nutrients in check with water changes.

If you've got a slimy, often blue-green or dark red sheet that coats the sand or covers surfaces like a film and peels off in one piece, that's cyanobacteria. It frequently carries a distinct musty or swampy smell. People call it "red slime" in saltwater and it's just as common in freshwater. The reason this matters: treat it like algae or just scrub it, and it won't hold.

Cyano is a photosynthetic bacteria, and it thrives in the exact conditions algae does plus one more: low flow and dead spots. It loves nutrient-rich water that's barely moving. So the fix is two-pronged. Physically remove what you can by siphoning the sheets out during a water change. Then fix the conditions. Improve flow so there are no stagnant zones (add or reposition a powerhead), cut nutrients with water changes and less feeding, and pull your photoperiod back.

There are "red slime remover" products that wipe out cyano fast, and they work. The most common one is Chemi-Clean. Boyd describes it as an oxidizer rather than an antibiotic, though its exact chemistry isn't publicly disclosed, so follow the label and run extra aeration either way. People sometimes reach for an erythromycin-based antibiotic like Maracyn instead, which targets bacteria more broadly; some hobbyists report a temporary impact on the biofilter, though the evidence is mixed, so monitor ammonia if you use it. Whichever route you take, if you don't fix the flow and nutrients the cyano comes right back, so we'd rather you fix the cause. If it's entrenched, come talk to us and we'll figure out whether a treatment is worth it for your specific tank.

Saltwater tanks play by the same rules, but reefers have sharper tools to measure and control them. Algae and cyano in a reef are almost always a phosphate and nitrate story. Test for both. If they're climbing, the usual culprits are overfeeding, an overstocked tank, or not enough export (your protein skimmer, water changes, and any media doing the work of pulling nutrients back out).

The single biggest upgrade for a chronic algae reef is RODI water. Tap water carries phosphate, silicate, nitrate, and other things algae loves, and you're pouring it in every top-off and water change. An RODI unit strips that out so you start from zero. Pair it with a quality salt mix and your baseline nutrient load drops hard. If you don't own an RODI unit, we carry them, and we also sell RODI water by the jug if you want to test the difference before committing.

From there: run phosphate-removing media (GFO) if your numbers stay high, keep your skimmer dialed in, and build a saltwater cleanup crew (trochus, astrea, and cerith snails, plus a few hermit crabs) to graze surfaces. One caution on lighting: a short 6-to-8-hour day is a fine short-term reset to knock algae back, but it's not a long-term target for a reef. Many LPS and SPS systems need a 10-to-12-hour photoperiod for coral health, so adjust based on your corals rather than cutting light to the bone. If you're fighting nuisance algae while trying to grow coral, see our guide on growing corals, because the same nutrient balance that starves algae is what keeps your corals colored up. Bring your test numbers in and we'll read them with you.

Once a tank is clear, keeping it clear is a short list of cheap habits. Put your lights on a timer, generally around 6 to 8 hours a day for a fish or low-tech planted tank (reefs run longer, set to your corals), and keep the tank out of direct sunlight. Feed once a day and only what the fish finish in a couple of minutes, because uneaten food is just dissolved algae fertilizer. Do regular water changes, usually 10 to 25 percent weekly depending on your stocking, to keep nitrate and phosphate from creeping up.

Keep a magnet cleaner or algae scraper on the tank and give the glass a quick swipe a couple times a week so nothing gets a foothold. Keep an appropriate cleanup crew working the surfaces. And test your water now and then, especially nitrate and phosphate, so you catch a nutrient creep before it becomes a green tank. A test kit pays for itself the first time it tells you why something's off.

That's it. No miracle bottle, no special light, no additive. Light on a timer, light feeding, water changes, a cleanup crew, and an eye on your numbers. If your tank keeps fighting you despite all that, something specific is off, and that's exactly what we're here for. Bring a sample to the shop and we'll test it and walk through what's feeding the problem.