Green water is caused by free-floating algae, not algae on surfaces

Green water happens when microscopic algae cells float throughout your tank instead of growing on glass or decorations. You cannot scrape it away because there is nothing solid to scrape. The water itself looks cloudy and green, sometimes bright like pea soup, and it blocks your view of the fish inside.

This algae blooms because of three things working together: too much light, excess nutrients (usually from uneaten food or fish waste), and time. Fix any one of these and the bloom collapses within days. Most aquariums clear in three to seven days once you start.

Key Takeaways

  • Green water is caused by free-floating algae that multiplies when your tank gets too much light and has too many nutrients.
  • The fastest fix is a combination of reducing light to 6 to 8 hours per day, doing a 25 to 50 percent water change, and removing uneaten food within two minutes of feeding.
  • An ultraviolet sterilizer kills the algae cells directly but costs $30 to $100 and is not necessary if you address the root causes.
  • Do not add chemicals or algaecides to a small tank—they can harm fish and create dead algae that makes the water worse before it gets better.
  • Once the water clears, keep it clear by feeding only what your fish finish in two minutes and limiting light to under 8 hours daily.

Reduce light to 6 to 8 hours per day when ready

Algae needs light to grow. If your aquarium light is on 10, 12, or 24 hours daily, cut it back to 6 to 8 hours. Set a timer so the light turns on and off at the same time each day—this also helps fish maintain a normal sleep cycle.

If your tank sits in direct sunlight from a window, move it away from that window or cover the window during peak daylight hours. Even a few hours of afternoon sun can fuel an algae bloom. If you cannot move the tank, use a shade cloth or curtain to block the light.

Reducing light alone will not clear the water in one day, but it stops the algae from multiplying faster than you can remove it. Combined with the other steps, it is the most important change you can make.

Do a 25 to 50 percent water change and clean the filter

Remove one-quarter to one-half of the water in your tank using a siphon or bucket. This removes some of the algae cells directly and dilutes the nutrients feeding the bloom. Use water that is the same temperature as your tank—cold water shocks fish.

While the water level is low, rinse your filter media (the sponge, cartridge, or floss inside the filter) in a bucket of the water you just removed. Do not rinse it under tap water, because tap water kills the beneficial bacteria living in the filter. Rinsing removes trapped waste that would otherwise break down into nutrients the algae feeds on.

Refill the tank slowly so you do not disturb the substrate or stress the fish. Do this water change once, then wait three days before doing another. Multiple changes in one day do not speed up clearing and can stress the fish.

Remove uneaten food within two minutes of feeding

Uneaten food sinks to the bottom and rots, releasing ammonia and other nutrients that algae blooms feed on. Feed your fish only what they will finish in two minutes. Watch them eat, and if food is still floating or sinking after two minutes, you fed too much.

Use a small net to scoop out any food that hits the bottom. Do this every time you feed. If you have a bottom-feeder like a pleco or corydoras catfish, they will eat some of it, but they cannot eat all the excess if you overfeed.

Overfeeding is the most common cause of green water in established tanks. Fish do not need much—a pinch of flakes or a few pellets per fish, once or twice daily, is enough. Hungry fish are healthier than overfed fish, and they produce less waste.

Use an ultraviolet sterilizer if the water does not clear in five days

A UV sterilizer is a small device that kills algae cells as water passes through it. You attach it to your filter output or place it in the tank, and it uses ultraviolet light to destroy the algae DNA. The water clears within 24 to 48 hours of running it.

UV sterilizers cost $30 to $100 depending on tank size and brand. They work well but are not necessary—most green water clears in three to seven days if you reduce light, do a water change, and stop overfeeding. Use one only if you have done those three things for five days and the water is still green.

If you buy a UV sterilizer, follow the manufacturer's instructions for flow rate and bulb replacement. The bulb loses power over time and needs replacing every 6 to 12 months, even if it still looks bright. A weak bulb will not kill algae.

Do not use algaecide or chemical treatments in small tanks

Algaecides and chemical treatments kill algae but create problems in small tanks. When the algae dies, it sinks and decays, releasing more nutrients and making the water cloudy and foul-smelling before it clears. This can also lower oxygen levels and stress or kill fish.

In large public aquariums with strong filtration, chemicals work because the filter can handle the dead algae. In a home tank under 50 gallons, the filter cannot process that much decay fast enough. You end up with worse water quality for several days.

Stick with light reduction, water changes, and removing excess food. These methods take a few more days but do not risk your fish or create a worse problem first.

Keep the water clear by preventing blooms in the future

Once your water clears, maintain it by keeping light to 6 to 8 hours daily and feeding only what your fish finish in two minutes. Do a 25 percent water change once per week to remove waste before it builds up. Clean the filter every two to four weeks in the removed water.

If you add new plants, they will compete with algae for nutrients and light, which helps prevent future blooms. Fast-growing stem plants like ludwigia and rotala are especially good at outcompeting algae. You do not need plants to keep water clear, but they help.

Check your light timer monthly to make sure it is working. A broken timer that leaves the light on 24 hours will cause a bloom within days, even if everything else is correct.

Frequently Asked Questions

Can I use a water clarifier to clear green water faster?

Water clarifiers clump algae particles together so they sink or get caught in the filter, but they do not kill the algae or stop it from growing back. They work for a few days, then the algae blooms again unless you also reduce light and nutrients. Skip the clarifier and fix the root cause instead.

Will adding more plants clear the green water?

Plants compete with algae for light and nutrients, so they help prevent future blooms, but they do not clear existing green water fast enough. Add plants after the water clears, not during the bloom. Fast-growing stem plants work better than slow-growing ones.

How often should I do water changes while clearing green water?

Do one 25 to 50 percent water change, then wait three days before doing another. Multiple changes in one day do not speed up clearing. After the water clears, do a 25 percent change once per week to keep it clear.

Is green water dangerous to my fish?

Green water itself does not poison fish, but it signals that nutrients are building up in the tank. If left unchecked, those nutrients can cause oxygen levels to drop at night, which stresses or kills fish. Clear it within a week to prevent that.

What if my aquarium light does not have a timer?

Buy a plug-in timer at any hardware store for $10 to $20. Plug the light into the timer, set it for 6 to 8 hours on and 16 to 18 hours off, and plug the timer into the wall. This is the easiest way to control light without remembering to turn it on and off by hand.