You walk past the tank one morning and the water has gone the colour of pea soup. The fish are still swimming about quite happily, the equipment is running, nothing smells wrong, and yet you can barely see the back glass. It is one of the more alarming things a new tank can do, mostly because it seems to arrive overnight and no amount of wiping the glass makes any difference. I have had it happen on new setups, on tanks that sat too near a window, and once on a tub of fry I was raising outdoors. The good news is that green water is one of the easier problems to fix once you understand what it actually is, and it very rarely hurts the fish.
What green water actually is
The green you are looking at is a bloom of single-celled algae floating freely in the water column. Aquarists call it phytoplankton or, less politely, green water. It is a completely different beast from the algae you scrape off the glass or find growing on decorations. That attached algae stays put; you can wipe it away. Green-water algae drifts, so every drop of water in the tank is faintly green, and there is nothing solid to remove.
The reason it appears so suddenly is that these organisms reproduce at a ferocious rate. Give them light and food and they can double their numbers in a day or two, which is why a tank that looked crystal clear on Monday can be opaque by Thursday. It is not dirt, it is not a filter failing, and it is not usually a sign that your fish are in danger. It is a population explosion, and to deal with it you have to think about what is feeding that explosion rather than reaching for the algae scraper.
What triggers a bloom
Two things drive green water: light and nutrients. Take either one away and the algae cannot sustain itself. The usual culprits are worth going through honestly, because the trigger is almost always something in the daily routine.
Excess light is the most common. A tank sitting in direct sunlight for part of the day is the classic case; even a couple of hours of sun through a window pumps in far more energy than any aquarium light. Long light periods do it too. If your lamp is on for ten or twelve hours because it is plugged into the same timer as the room lamp, that is plenty.
Nutrients are the other half. Nitrate and phosphate, the leftovers of feeding and fish waste, are algae fertiliser. An overstocked tank, a heavy hand at feeding time, or water changes that have quietly slipped from weekly to whenever-you-remember will all push those levels up. A brand-new tank is especially prone, because the filter bacteria are not established yet and small ammonia spikes act like a starting gun for algae. They respond to that ammonia far faster than your beneficial bacteria can. If your tank is new and going green, it may still be finding its feet; my guide to cycling a new aquarium explains what is happening under the surface.
Why the obvious fix does not work
Everyone’s first instinct is a big water change, and it makes sense: dirty-looking water, so replace it. The trouble is that green-water algae multiplies faster than you can dilute it. You can drain half the tank, and within a day or two the survivors have bred the water green again. I have watched people do daily 50 percent changes for a week and get nowhere, growing steadily more frustrated. Water changes are part of the long-term answer, but on their own they are like bailing a boat without finding the leak.
The bottled clarifiers and flocculants sold for this clump the algae together so the filter can catch it. Sometimes they buy you a day of clearer water, but if the underlying cause is still there the bloom simply returns. They treat the symptom.
The blackout method
This is my usual first move because it costs nothing. Algae needs light, so you take the light away completely. Turn the lamp off, then wrap the whole tank in a couple of thick towels, a blanket, or cardboard so that no daylight gets in either. Leave it fully covered for three to four days. Keep the filter and any air pump running the entire time, because the fish still need oxygen and flow.
Do not feed during the blackout, or feed only a tiny amount. Healthy adult fish come to no harm going a few days without food, and any food you add just becomes more nutrient for the algae. When you uncover the tank, the water should be noticeably clearer, sometimes almost clear. Follow it straight away with a large water change to remove the dead algae and the nutrients that fed the bloom. The plants sulk a little from the darkness but recover within a few days.
Blackouts work because fish and plants tolerate a short spell of darkness far better than a fast-breeding alga can. It is a bit of a nuclear option in that you cannot enjoy the tank while it is wrapped up, but it is effective and free.
A UV steriliser, the reliable cure
If green water is a recurring problem, or you simply want the quickest fix, a UV steriliser is the tool that ends the argument. Water is pumped through a chamber where an ultraviolet lamp kills the free-floating algae as it passes. Because green-water algae lives suspended in the flow, nearly all of it gets carried through and destroyed. Most tanks clear within one to three days of running one.
Two honest caveats. A UV unit only affects what actually passes through it, so it does nothing for algae attached to the glass or decorations, and it will not fix the underlying nutrient problem. It is a cure for the bloom, not the cause. It is also an extra piece of kit to buy and plumb in. That said, once you own one it is permanent insurance; I keep a spare in the cupboard and it has rescued more than one tank of mine. I would not tell a beginner they must buy one, but if you fight green water more than once, it is money well spent.
Stopping it coming back
Clearing the water is only half the job. If you do not change what caused the bloom, it will return. Work through the triggers one at a time.
Cut the light first. Move the tank out of direct sunlight if you possibly can, or shade the affected side. Bring the lighting period down to six or seven hours a day on a timer. Aquarium plants do fine on that; algae struggles.
Then tackle the nutrients. Feed less, and only what the fish clear in a couple of minutes. Get water changes back onto a steady weekly rhythm; my water change calculator works out a sensible volume for your tank size, and there is more on the why and how in my guide to water changes. Check you are not overstocked, since more fish means more waste; the stocking calculator gives you an honest read on that.
Live plants are the quiet long-term fix. A tank with plenty of fast-growing plants has to share its nitrate and phosphate with them, and healthy plants are very good at starving algae of the food it needs. If your tank is sparsely planted, a few undemanding stems will help; my beginner plants guide covers species that grow without fuss. It is also worth reading up on algae in general, because the same habits that prevent green water keep the glass cleaner too.
When green water is not a problem at all
One last thing, because it surprises people. Green water is not always the enemy. Fish breeders deliberately culture it to feed baby fish and tiny invertebrates like daphnia; those free-floating algae are an excellent first food for fry. So if you ever find yourself raising a batch of livebearer babies and the water goes green, you may have accidentally created the perfect nursery. In the display tank it is an eyesore you want gone. In a fry tub it can be a happy accident. Context is everything, and knowing which situation you are in tells you whether to reach for the blackout towels or just leave it be.