Every planted tank grows some algae. Every single one. What separates a tank that looks clean from a tank that looks overrun is rarely effort — it is whether the conditions quietly favour plants or quietly favour algae. The good news is that algae is honest. Each type tends to show up for a recognisable reason, which means the growth on your glass is less of a mystery and more of a message. Read the message first, and the fix usually becomes obvious.
Algae is a balance problem, not a dirt problem
Plants and algae want the same three things: light, carbon and nutrients. The difference is how much of each they need, and how fast they can use it. Algae are opportunists with tiny requirements; your plants are the heavy feeders. Algae takes over when the tank offers more of one ingredient than the plants can process with the other two — most often, more light than the available CO2 and nutrients can support.
Think of it as a three-legged stool:
- Light sets the speed of everything. More light (brighter, or on for longer) means plants need more carbon and more food to keep up.
- CO2 is the carbon supply. In a low-tech tank it comes only from the air and the livestock, so it is limited. In an injected tank, what matters most is that the level is stable while the lights are on — many keepers target roughly 30 ppm, checked with a drop checker.
- Nutrients are the plant food: nitrate, phosphate, potassium and trace elements. Too little starves the plants; wild swings in supply unsettle them. Healthy planted tanks commonly run nitrate somewhere around 10–20 ppm — deliberately not zero.
There is a quiet fourth factor underneath all three: consistency and flow. Plants adapt to the conditions they are given, but they pay for every swing — lights on for twelve hours one day and six the next, CO2 strong in the morning and gone by afternoon. Water movement matters too, because flow is what delivers CO2 and nutrients evenly to every leaf and carries waste away to the filter. The corners where flow barely reaches are exactly where organic debris settles, and exactly where the tougher algae usually start.
When one leg gets longer than the others, plants stall and algae — which needs far less to get going — moves into the gap. That is why the fixes in this guide are almost never “scrub harder.” They are “shorten the long leg, or lengthen the short ones.” If you want the full picture on the carbon leg, our guide to CO2 for planted tanks walks through it step by step.
Step one: put a name to it
You cannot fix what you cannot name, and different algae point at different causes. Work through the table below with your tank in front of you. Colour, texture and — just as important — where it is growing will narrow it down quickly.
| Type | What it looks like | Where it grows | Usual trigger |
|---|---|---|---|
| Brown diatoms | Dusty yellow-brown film; rubs off easily between your fingers | Glass, substrate, plant leaves, decor — everywhere, in young tanks | A new, still-maturing tank; diatoms build their cell walls from silicates and thrive while the filter and microbes are still establishing |
| Green spot algae | Small, hard, bright-green dots that resist a fingernail | Glass and the slow-growing leaves of plants like Anubias | Too little phosphate relative to the light, often under strong lighting |
| Green hair / thread algae | Fine green strands and fuzz, from a few millimetres to long threads | Plant leaves, moss, wood and rock edges | More light than the plants can match with CO2 and nutrients; often after a light increase or in a sparsely planted tank |
| Black beard algae (BBA) | Short, dark, bristly tufts — actually a red algae, despite the colour | Leaf edges, wood, rock, filter outlets and equipment | Unstable or low CO2, build-up of organic waste, and dead spots with weak flow |
| Staghorn algae | Coarse, branching grey-green strands, like tiny antlers | Leaf edges and hardscape, often near the filter outflow or in low-flow corners | Ammonia spikes, low CO2 and poor circulation |
| Blue-green algae (cyanobacteria) | Slimy blue-green (sometimes dark) sheets with an earthy, musty smell; peels away in mats | Across the substrate surface and along the front glass at the substrate line | Low nitrate, poor circulation and accumulated organic waste; it is a bacterium, not a true algae |
Two quick reality checks before you act. First, a young tank with a brown dusting in its first month is almost always diatoms, and that is a normal stage of a tank maturing rather than a failure. Second, if the growth is slimy, sheet-like and smells earthy when you lift a corner, treat it as cyanobacteria — the playbook for it is different from the green algae.
Step two: the fix playbook, type by type
Every fix below follows the same shape: remove what is already there, correct the trigger, then wait. Skipping the middle step is why algae “always comes back.” One habit supports all six fixes: a weekly water change of roughly a quarter to a half of the tank. It physically exports dissolved waste and the algae you have loosened, resets nutrient levels before they drift, and costs nothing but a bucket and ten minutes. If your routine currently has no regular water change, start there — it is the highest-value change most beginners can make.
Brown diatoms: mostly, wait it out
Diatoms typically appear within the first two to three weeks of a new setup and fade on their own over the following weeks as the filter matures and other microbes establish. Wipe the glass, siphon the dust off leaves and substrate during your normal water changes, and keep the routine boringly consistent. Do not chase them with products. Only if a heavy brown film is still going strong after a couple of months is it worth testing your tap water for high silicates.
Green spot algae: check phosphate and light
Hard green dots on glass and slow leaves are most often linked to phosphate running low relative to the light the tank receives. Scrape the glass with an algae scraper or an old bank card. If you dose fertiliser, check whether your routine actually includes phosphate — many beginners dose everything except it. If the light is strong or the day is long, trim the photoperiod back toward 6–8 hours. A few spots on an old Anubias leaf are normal; a fast-spreading crust is the signal to act. Our planted tank fertilizer guide explains how the nutrients fit together.
Green hair and thread algae: rebalance, then harvest
Twirl strands out with a clean toothbrush, wind longer threads around it like spaghetti, and siphon up the loose bits during a water change — what you leave behind keeps growing. Then address the cause: this algae usually means light is outrunning CO2 and nutrients. Reduce the photoperiod, raise or dim an intense light if you can, and make sure your plants are actually being fed. A tank with fast-growing, well-fed plants is the long-term answer; algae struggles to compete with healthy plant mass.
Black beard algae: stabilise the CO2, clean the organics
BBA is the stubborn one, and it is stubborn because its trigger is usually instability rather than a single missing ingredient. Work through the basics in order: keep CO2 steady for the whole lighting period if you inject it (same timing every day, on before the lights); improve flow so there are no dead zones around the tufts; and cut the organic load — vacuum the substrate, clear decaying leaves, and rinse filter media in removed tank water. Prune leaves that are more beard than leaf, and scrub tufts off wood and rock with a toothbrush during water changes — BBA grips hard and rarely lets go on its own, so what you do not remove mostly stays. Tufts colonising filter outlets and equipment are worth cleaning too, since they seed the rest of the tank. Some keepers spot-treat individual tufts with a liquid carbon product applied directly with a syringe, following the product label dose exactly; treated BBA typically pales and dies back over the following days. Be careful here — overdosing liquid carbon can damage sensitive plants and livestock, and it only kills the tuft you touch. It never fixes the cause.
Staghorn algae: flow, and hunt the ammonia source
Those stiff little antlers often follow an ammonia spike or chronically low CO2 with weak circulation. Remove strands by hand (they do not let go willingly, so trim badly affected leaf tips), check that nothing is rotting in a corner or clogging the filter, and improve flow through the area where it appears. If the tank is new or you recently disturbed the substrate a lot, give the filter time to recover and test ammonia to confirm it reads zero.
Blue-green algae (cyanobacteria): the reset routine
Peel and siphon off every sheet you can reach — it lifts away in mats, which makes manual removal genuinely effective here. Then fix its favourite conditions: raise low nitrate into the normal planted-tank range with your fertiliser routine, improve circulation along the substrate, and clear mulm from the surface layer. Keep the light period at the shorter end while you do this. Sheets that return within days are telling you the cause is still there; sheets that return more slowly, then stop, mean the reset is working.
Step three: change one thing, then give it two weeks
Here is the rule that saves beginners the most money and frustration: make one meaningful change, then watch for about two weeks before judging it. Algae responds slowly. The growth you see today started days ago, and existing algae does not vanish the moment conditions improve — it stops spreading first, then weakens, then grazes and water changes clear the rest. New plant growth is your scoreboard: clean new leaves mean the fix is working, even while old algae is still being cleared.
In practice, progress usually looks like this: in the first week, cleaned surfaces stay clean for longer and the growth you removed does not rush back. By the second week, new algae is visibly thinner and slower, and plant tips are pushing out clean leaves. That is the moment to hold your nerve and keep the routine steady rather than declaring victory and relaxing it — the tank needs a few more consistent weeks to lock the new balance in.
Changing the light, the CO2 and the fertiliser all in the same weekend feels productive, but it destroys the information you need. If things improve, you will not know why; if they worsen, you will not know what to undo. One change, two weeks, observe. Repeat. Boring, and it works.
Clean-up crew: welcome help, not a cure
Some livestock genuinely graze algae, and a well-chosen clean-up crew keeps on top of the soft everyday film so you do not have to. Amano shrimp are enthusiastic grazers of hair algae and soft green growth. Nerite snails work glass and hardscape steadily and will not breed in fresh water. Siamese algae eaters are one of the few fish that will pick at black beard algae, though they grow larger than many beginners expect, so check they suit your tank size and stocking before adding them.
Stock these animals because they fit your tank and you enjoy keeping them — never as a treatment you pour in and wait on. No grazing animal out-eats an imbalance, and a crew added to a tank with the wrong conditions simply becomes more livestock living in the wrong conditions. Fix the balance first; let the crew handle the dusting.
What not to do
- Do not reach for algaecide first. Chemical algae killers treat the symptom you can see and ignore the imbalance you cannot. The algae dies, the cause remains, and the algae returns — meanwhile some products stress plants and invertebrates. Cause first, always.
- Do not misuse blackouts. Wrapping the tank in darkness for days is a drastic reset sometimes used against green water or cyanobacteria. Used casually against hair algae or BBA it mostly punishes your plants, which need the light too, while the trigger stays untouched. It is a last-resort tool for specific problems, not a routine.
- Do not strip the filter. Replacing all the filter media or scrubbing it under tap water destroys the beneficial bacteria your tank depends on, and the resulting ammonia wobble is exactly the sort of instability that feeds staghorn and BBA. Rinse media gently in water you have removed from the tank, and stagger any media changes.
- Do not starve the tank. Cutting fertiliser to zero because “nutrients feed algae” backfires: starved plants stall, and stalled plants are algae’s best opportunity. Feed the plants properly and control algae with light and balance instead. If your setup runs without injected CO2, our low-tech planted tank guide shows how to set the balance for that style of tank.
- Do not expect a spotless tank. A thin film on the glass between cleanings and a little grazing growth on old wood are signs of a living tank, not a failing one. Aim for plants clearly winning — not sterility.
Identify, correct the cause, remove by hand, wait two weeks. Run that loop calmly and almost every common algae becomes a short chapter in your tank’s story instead of the whole book.