04 Aquarium
Algae in a goldfish tank, and green water
Every lit tank with fish in it grows algae. The useful question is not how to eliminate it but which kind you have, because each one is telling you something different about the light and the nutrients.
Every aquarium that has light falling on it and fish living in it grows algae. That is not a failure of maintenance; it is what happens when you put nutrients and light in the same box. A goldfish tank grows more than most, for reasons that are structural rather than careless, and the useful goal is management rather than elimination.
The fastest way to get somewhere is to identify what you have. Each kind grows under different conditions, so the algae is already telling you where to look.
What you are looking at
| What you see | Usually means | What helps |
|---|---|---|
| Hard green spots on the glass, hard to scrape | Strong light, a mature tank | Scraping; it is normal and mostly cosmetic |
| Soft green film or dust on the glass | Light and nutrients together | Shorter photoperiod, more water changes |
| Green threads or hair on decor and plants | Nutrients running ahead of uptake | Manual removal by twirling, then feed less |
| Brown dusty film, new tank | Diatoms, common in the first months | Usually passes as the tank matures |
| Dark tufts like wiry beard on leaf edges | Persistent, tied to unstable flow and organics | Remove affected leaves and decor |
| Blue-green slimy sheets, musty smell | Cyanobacteria, not an alga at all | A different problem; see below |
| Cloudy green water, surfaces clean | Free-floating algae bloom | Blackout or UV; see below |
What actually reduces it
Fewer hours of light. Six to eight on a timer is enough for the fish and for viewing, and it is the first lever to pull. Ten or twelve hours is an algae culture. The lighting guide covers the photoperiod in more detail.
No direct sunlight. A tank beside a window gets many hours of very strong light that no timer controls, and it warms as well. Moving the tank, or closing a blind for part of the day, often does a great deal.
Less food. Uneaten food and heavier feeding both raise the nutrients directly. This is a lever many keepers are reluctant to pull, and it has a clear effect.
More water changes. Nitrate only leaves the tank in the water you remove. If nitrate is sitting above 40 mg/L, the algae has a supply that nothing else will interrupt; the water change guide sets out the rhythm.
Plants, where they survive. Anubias, java fern and other tough species tied to wood or rock compete for the same nitrogen and are unpalatable enough to last. The plants guide covers which ones stand a chance.
And accepting some. A film on the back and side glass is a perfectly good outcome. Scrape the viewing pane, leave the rest, and let the fish graze it.
Removing it by hand
The unglamorous half of the answer, and the tools matter more than people expect.
The glass. A magnet cleaner for the weekly pass, a blade scraper for anything hard enough to resist it. The rule that matters: keep both away from the substrate. A magnet that picks up one grain of sand draws a long, permanent scratch across the pane in a single stroke. Work from the top down, stop a few centimetres above the sand, and do that last strip by hand with a soft pad or the edge of an old credit card.
On acrylic, no blade and no hard-faced magnet at all. Acrylic scratches from almost anything, and a scratched acrylic pane cannot be polished out at home. Soft pads only.
Green spot algae — the hard little discs — usually needs a blade on glass, and is the one kind most worth simply tolerating on the back and sides.
Hair and thread algae comes out by twirling: a toothbrush, a skewer or a fork turned slowly winds the strands up like spaghetti and lifts them out in one piece. Pulling breaks it and scatters fragments that regrow.
On decor, take the piece out and scrub it in a bucket of tank water, or in plain water with a brush kept for the purpose. Never soap.
And siphon as you go, because everything you scrape ends up in the water column and then in the substrate — the water change guide covers doing this as part of the weekly job rather than as a separate one.
Algae on plants
A different problem, because you cannot scrape a leaf.
What it means. Algae growing on plant leaves is usually a sign that the plants are growing more slowly than the algae — too little light for the plants, or not enough of the nutrients they specifically need, while the nitrogen the algae wants is abundant. In a goldfish tank that is the normal state of affairs, which is why slow-growing anubias and java fern often carry some.
What to do:
- Shorten the light rather than increasing it. More light favours the algae, which grows faster than anubias by a wide margin.
- Check the plants are actually fed. Potassium, iron and trace are what runs short here, never nitrogen — plants for goldfish tanks covers fertilising in a tank full of fish waste.
- Wipe the leaf gently between finger and thumb for soft films. Tough anubias leaves tolerate it.
- Remove badly affected leaves at the stem. The plant will replace them and the rhizome is unaffected.
- And leave a little. Goldfish graze leaf films, which is one of the few natural foraging behaviours a tank offers.
The sequence in a new tank
Worth knowing so you do not treat a phase as a problem.
Typically weeks two to eight: brown diatoms, the “brown algae” of a new tank. A dusty brown film over glass, substrate and decor, thriving on the silicates and unstable conditions of a young tank. It looks unpleasant, it wipes off easily, and it fades on its own as the tank matures. This is a common “something is wrong with my new tank” and the answer is a cloth and patience.
From about the second month: a green film as the tank settles and the light becomes the dominant factor. This is the point at which the photoperiod is worth setting properly.
After that: a stable low level, mostly on the back and sides, that a weekly wipe of the viewing pane keeps in hand. The timings are keepers’ experience, not measurements, and vary from tank to tank.
What disrupts the sequence is reaching for treatments during it. An algaecide in a cycling tank is a chemical insult to a bacterial colony that is still establishing, and it is how a manageable phase becomes a real problem — cycling a goldfish aquarium covers what that colony needs, which is mostly to be left alone.
Green water
Green water is a different phenomenon and deserves its own treatment. It is a bloom of single-celled algae living suspended in the water rather than attached to a surface, so the glass may be spotless while the water is pea soup.
Which cloudy water you have
-
Milky white
A bacterial bloom. In a new tank the haze usually clears by itself; keep testing ammonia and nitrite.
-
Green
Free-floating algae: a different problem.
-
Tea-coloured, clear
Tannins from wood. Harmless.
-
Brown, opaque
Stirred substrate. It settles.
-
Settles overnight
Dust from unrinsed sand. Not biological.
It is not toxic, but a dense bloom can leave the water short of oxygen before dawn and push the pH high in the afternoon, especially in soft water, and a sudden die-off removes oxygen too, so keep the surface moving. Pond keepers have historically welcomed it: it shades the fish and feeds the microscopic life that fry eat. The other problem in an aquarium is that you cannot see the animals, which matters for more than aesthetics — most of the early warnings a goldfish gives you are visual.
Water changes alone are the usual first attempt and usually fail. Removing half the water removes half the cells, and the remainder divide back into the space within days, because the light and the nutrients that produced the bloom are both still there.
What does work:
- A blackout. Many keepers cover the tank completely for three to five days — no light at all, not even room light through the glass. The suspended algae dies off, and goldfish generally cope with a few days of darkness. Keep feeding light and keep the filter and aeration running.
- A UV clarifier. Water passing through a UV unit has the free-floating cells killed as it goes. It is the reliable fix, does nothing to algae on surfaces, and is the standard answer for ponds rather than for tanks — the UV buying guide sets out why, and how to size one for a pond.
- Then remove the cause. Cut the photoperiod, deal with the window, and bring the nutrients down, or the bloom will simply return.
A blackout, step by step
The cheapest fix for green water and easy to do badly.
- Cover the tank completely. Lights off, and a blanket, black bin bags or cardboard over the whole thing so no daylight or room light reaches the water. Partial darkness does not work; the algae only needs a little.
- Leave the filter and air pump running the entire time. This is not optional — the fish need the oxygen and a large algae die-off will consume more of it.
- Feed very lightly, or not at all. A few days without food is generally nothing to a healthy adult goldfish, and less food means less of the nutrient that caused this.
- After about three to five days, uncover.
- Water change straight away, and siphon what has settled.
- Clean the mechanical media a few days later, once the filter has collected the dead cells. Not the same day as the water change, and not the biological media.
- Then fix the cause, or the bloom is likely to return.
When not to do it: during a cycle, while a fish is unwell, or in a heavily stocked tank without extra aeration. Darkness itself is rarely the problem; an oxygen dip can be.
Blue-green slime is not algae
Cyanobacteria look like algae, are usually called blue-green algae, and are bacteria. They grow as slimy sheets over substrate, decor and plants, peel off in one piece, and often smell musty. Keepers also tend to find them where flow is poor and organics are high — the dead corner behind a filter outlet, the patch of substrate nothing disturbs.
They respond to different handling: keepers often use a full blackout, and the durable fix is usually improving circulation in the stagnant area and reducing the organic load that fed it. Scraping alone commonly fails, because a fragment left behind can regrow quickly.
Where the phosphate comes from
The other half of the nutrient pair, less discussed than nitrate and worth knowing about.
Mostly from food. Fish food is phosphate-rich, and what is not absorbed is excreted or left to decay. This is why “feed less” appears on every list of algae remedies — it cuts both nutrients at once, which nothing else does.
Sometimes from the tap. Some water suppliers dose orthophosphate to reduce lead leaching from old pipes, at levels that are irrelevant to human health and enough to feed an aquarium’s algae. If your tank is difficult and everything else has been done, this is worth checking — the source water page covers finding out what is in your supply.
Occasionally from the decor, if something in the tank is dissolving.
Should you test for it? Rarely. Phosphate kits are sold widely and the reading rarely changes what you would do, because the action in a goldfish tank is the same either way: less food, shorter light, more water changes. The testing guide lists it among the tests not worth buying for this hobby.
And phosphate removers — the resins and pads — treat a symptom in a tank that is producing the nutrient continuously. They work, they are a consumable cost, and they do not change why the phosphate is there.
Algae outdoors
Briefly, because a pond plays by different rules and has its own pages.
Green water in a pond is often welcome, or at least tolerated: it shades the fish and feeds the microscopic life fry depend on. The complaint is that you cannot see them, and a dense bloom can leave the water short of oxygen before dawn. A UV clarifier is the usual fix and shade plus planting is the durable one — goldfish ponds covers the trade-off, including the point that a pond polished clear by UV is not the healthier pond.
Blanketweed is the stringy green mat that grows on pond surfaces and is the outdoor version of hair algae. It is pulled out by twirling, exactly as indoors, and a pond through the year covers the seasonal pattern and what actually reduces it.
And the shared danger is the same one as the Callout above: a large mass of algae dying at once, whether from a treatment or a sudden clearing, takes the oxygen with it. Outdoors that can crash a pond’s oxygen fast, which is why gasping at dawn after a pond suddenly clears is a recognised emergency.
When algae is doing you a favour
Worth gathering, because the whole page reads as a list of remedies and the target is not zero.
It is food, and it is the right kind. Goldfish graze soft films continuously, and that grazing is one of the few natural foraging behaviours a tank allows an animal built to spend its day looking for things to eat — goldfish behaviour and intelligence covers why occupation matters, and goldfish welfare covers it as a welfare question rather than an aesthetic one.
It produces oxygen through the day, though it uses some at night, and takes up nitrate while it grows.
It is part of the biofilm that matures a tank. The same thin layer that coats the glass coats the decor and the substrate, and it supports the small organisms and the snails that process leftovers — snails, hitchhikers and biofilm covers that population and why most of it is welcome.
And a green back wall on a clean front pane is a good outcome, not a compromise. The tank looks better for it, the fish have something to do, and you have spent the time on water changes instead of scraping.
The only kinds worth pursuing hard are the ones that signal something — a sudden bloom, blue-green slime in a dead corner, or green water you cannot see through.
The short version
Work out which kind you have, shorten the light before anything else, get the tank out of the sun, feed less and change more water. Treat green water with a blackout or a UV unit rather than buckets, keeping the surface moving, and treat blue-green slime as a different problem from algae, one that often goes with poor flow. Then let the rest be: a goldfish tank with some algae in it is a goldfish tank, and the fish will thank you for the grazing.
Frequently asked questions
Is algae bad for my goldfish?
In itself, no. Algae is not toxic, it produces oxygen during the day, and goldfish graze on the soft kinds with evident enjoyment. A tank with a green back wall and clean glass at the front is a perfectly healthy tank. What matters is what the algae is reporting: a heavy, sudden bloom usually means nutrients have climbed or the light is on too long, and both of those are worth knowing about for reasons that have nothing to do with how the tank looks. The exceptions are a dense bloom of green water, which can leave the water short of oxygen before dawn, and an algaecide-driven die-off, where a large mass of algae decaying at once can pull the oxygen down sharply.
Why has my tank water gone green and cloudy?
That is green water, a bloom of single-celled algae suspended in the water rather than growing on a surface. It is set off by strong light — very often daylight from a nearby window — combined with plenty of dissolved nutrients, which a goldfish tank always has. It is not toxic and pond keepers have traditionally welcomed it, but a dense bloom can leave the water short of oxygen before dawn and push the pH high in the afternoon, especially in soft water, so keep the surface moving. In an aquarium it also hides the animals you are supposed to be watching. Water changes alone rarely clear it, because the cells that remain simply multiply back into the space you made.
What is the best way to scrape the glass?
A magnet cleaner for the weekly pass and a blade for the hard spots, with one rule that matters more than either: keep both away from the substrate. A magnet that picks up a single grain of sand will draw a long scratch across the glass in one pass, and the scratch is permanent. Start at the top, stop a few centimetres above the sand, and finish that last strip with a soft pad or a credit card by hand. On acrylic, no blade and no magnet with a hard face at all — acrylic scratches if you look at it firmly.
My new tank has gone brown. Is something wrong?
Almost certainly not. A brown dusty film in the first weeks or months is diatoms, which thrive on the silicates and unsettled conditions of a young tank and fade on their own as it matures. Wipe the viewing glass and otherwise leave it. The usual sequence in a new tank is brown diatoms first, then a green film as the tank settles, and then a stable low level that a weekly wipe keeps in check. Reaching for treatments during that sequence is how people create a second problem.
How do I do a blackout properly?
Keepers usually give it three to five days of total darkness: lights off, and the tank wrapped in something opaque so no room light or daylight reaches it. Keep the filter and any air pump running throughout, feed very lightly or not at all, and do not do it during a cycle or while a fish is unwell. Uncover it on the last day, do a water change, clean the filter's mechanical media a few days later once the dead algae has been collected, and then fix the cause — a blackout that is not followed by a shorter photoperiod often buys only a few weeks.
Will an algae eater or a pleco solve it?
Adding an animal to a tank never reduces its nutrient load; it raises it. The common pleco reaches 40 cm and wants tropical warmth, the Chinese algae eater grows large, loses interest in algae and is known for rasping the flanks of slow fancy goldfish, and almost every small algae-eating species is either eaten or chronically cold in goldfish water. The tankmates guide covers which ones genuinely work. The things that reduce algae are fewer hours of light, less food, more frequent water changes and, where they survive goldfish, live plants.