We are reader supported. When you purchase through links on our site, we may earn an affiliate commission. Also, as an Amazon affiliate, we earn from qualifying purchases.
If the water in your aquarium looks like pea soup, you are dealing with “green water.” It can be surprising and even a little scary for new fish keepers, but the good news is that green water is common and fixable. In this guide, you will learn what green water is, why it happens, how to clear it quickly, and how to prevent it for good. The steps are beginner-friendly, safe for your fish, and based on real aquarium experience.
What Exactly Is Green Water?
Green water is caused by tiny, free-floating algae called phytoplankton. These are not the same as hair algae or algae stuck on your glass or decorations. Because they float in the water column, they give the whole tank a green, cloudy color. This is why the water can look like green tea even if your glass and rocks look clean.
Free-floating algae multiply fast when they get what they like: lots of light and lots of nutrients. Once they bloom, normal filters often cannot catch them because the particles are extremely small. That is why the water can stay green even when your filter is running and the tank seems “clean.”
Is Green Water Dangerous?
Most of the time, green water is more of a visual problem than an immediate danger. Many fish live through it just fine. However, very heavy blooms can reduce oxygen at night, stress sensitive fish, and block light from reaching your plants. Green water also signals an imbalance in your tank. Fixing the cause will make your aquarium healthier overall, not just clearer.
Why Did My Aquarium Turn Green?
Too Much Light
Algae love light. If your tank sits near a sunny window or your aquarium light runs for 10 to 14 hours a day, algae can take off. Even a few hours of direct sunlight can trigger a bloom. Strong new lights or a long photoperiod on a fresh setup can also be a trigger.
Excess Nutrients
Algae need food. They eat dissolved nutrients like ammonia, nitrate, and phosphate. Overfeeding, crowded tanks, dead plant leaves, and infrequent cleaning add nutrients to the water. Tap water can also bring in phosphate and nitrate. When nutrients build up, algae have everything they need to multiply.
New Tank Syndrome
In new aquariums, the beneficial bacteria that process fish waste are not fully grown yet. Ammonia and nitrite can swing. Algae can take advantage of this unstable period, especially if the light is strong or the tank gets sunlight. Green water blooms are common in the first few months if the tank is not balanced.
Filter or Flow Issues
Weak filtration, clogged media, or poor water movement can lead to dead spots where waste collects. As that waste breaks down, it feeds algae. If you clean filter media in tap water and kill good bacteria, you can trigger mini-cycles and algae blooms.
Sudden Changes
Big water changes with nutrient-rich tap water, switching lights, heavy pruning, or deep cleaning can upset the balance. Sometimes a bloom follows after a big change because algae adjust faster than plants or bacteria.
How to Confirm It’s Green Water
Look at the Water
Green water looks evenly green or yellow-green throughout the tank. You cannot wipe it away like algae on glass. Shining a flashlight through the tank makes the color obvious. If it is green when viewed from the side, you are dealing with free-floating algae.
Test Your Water
Use a liquid test kit to check ammonia, nitrite, nitrate, and pH. If ammonia or nitrite is above zero, your biofilter is struggling. If nitrate is very high (over 40 ppm), there is likely nutrient buildup. A phosphate test is also helpful. If your tap water has high nitrate or phosphate, blooms are more likely after water changes.
Review Your Setup
Ask yourself how long your lights are on, whether the tank gets sunlight, how much you feed, how often you change water, and how you clean your filter. This quick audit often shows the main cause.
Quick Ways to Clear Green Water
There are fast techniques that can make your water clear again while you fix the root causes. You can use one of these or combine them depending on your situation.
The Blackout Method (2–4 Days)
The blackout method deprives algae of light, which stops them from photosynthesizing and reproducing. To do it safely, turn off the tank light and cover the aquarium with a thick blanket or towels to block all light. Do not feed heavily during this period, and keep the filter and air pump running for oxygen. After 2 to 4 days, uncover the tank, do a 30–50% water change, and clean the glass. The water should be noticeably clearer. This method is safe for fish and most plants for a short period.
UV Sterilizer (Most Reliable Clear-Up)
A UV sterilizer passes water past a UV light that kills free-floating algae cells as they flow through. It does not hurt fish or plants when used correctly. It is very effective for green water and often clears the tank within a few days. For small aquariums up to about 30 gallons, a 5–9 watt unit often works. For 40–75 gallons, 9–18 watts is common. Place it after mechanical filtration if possible, and run it continuously until the water is clear and stable. UV removes the algae bloom but does not remove the nutrients, so you still need to address the cause.
Diatom or Micron Polishing Filters
Fine polishing filters or diatom filters can physically trap the tiny algae cells. These can clear the water fast but may clog quickly when the bloom is heavy. They are a great short-term tool if you have access to one, especially in larger tanks.
Flocculants and Clarifiers
Water clarifiers cause tiny particles to clump together so your filter can catch them. They can help, but they do not fix the cause and may stress gills if overdosed. If you use a clarifier, follow the label precisely and keep extra mechanical media in your filter to catch the clumps. Remove or rinse it after a day so trapped waste does not rot in the filter.
Large Water Changes (With Care)
Water changes reduce nutrients, which is helpful, but algae can return fast if the underlying issues remain. If your tap water has high phosphate or nitrate, big water changes can even make things worse. Use a good dechlorinator. If your tap water is nutrient-rich, consider using a phosphate remover pouch in your filter or blending in some reverse osmosis (RO) water if practical for you.
Long-Term Prevention: Fix the Balance
To keep green water from coming back, focus on balance. Algae need light and nutrients. Control both, and they cannot bloom.
Set a Reasonable Photoperiod
Run your aquarium light for 6 to 8 hours per day for most tanks. Use a simple plug-in timer so it is consistent. If you have strong lighting or are battling algae, start at 6 hours and increase slowly only if plants need more. Avoid direct sunlight. If the tank is near a window, close blinds during the brightest hours or move the tank if possible.
Adjust Light Intensity
High-intensity lights can overwhelm a low-tech tank (no added CO2, few plants). If your fixture has dimming, reduce brightness. If it does not, raise the light higher above the tank or add floating plants to shade the water surface.
Control Feeding
Overfeeding is the easiest way to cause excess nutrients. Feed only what your fish eat in about 30 seconds to 1 minute, once or twice a day. Remove leftover food with a net. For new tanks, feed lightly until the biofilter is stable.
Regular Water Changes and Gravel Cleaning
Do a weekly water change of 30–50% for most community tanks. While changing water, vacuum the substrate to remove fish waste and decaying plant matter. Clean your glass as needed to remove surface algae, which can reduce light entering the tank.
Keep the Filter Healthy
Rinse filter sponges or pads in old tank water, not under the tap, to protect beneficial bacteria. Do not replace all filter media at once. Maintain good water flow so there are no dead spots collecting waste. A small internal circulation pump can help in larger tanks.
Use Live Plants as Competition
Healthy live plants use the same nutrients algae want. Fast-growing plants such as hornwort, water sprite, Hygrophila, and stem plants help a lot. Floating plants like salvinia, frogbit, or water lettuce are especially good because they shade the tank and use nutrients quickly. If you keep plants, ensure they have enough nutrients and moderate light so they thrive. Weak plants shed leaves and can make algae worse.
Consider Phosphate Control
If your tap water has high phosphate, algae blooms are more likely. You can run a small pouch of phosphate-removing media in your filter. Replace it as directed. This is optional but helpful in tough cases.
Stocking and Bio-Load
Too many fish in a small tank means more waste and more nutrients. Make sure your aquarium size matches your fish and do not add too many at once. As a simple guideline, add new fish slowly and give your filter bacteria time to grow.
Special Notes for Planted Tanks
Balance Light, CO2, and Nutrients
In planted aquariums, strong light without enough CO2 or nutrients often causes algae. If you do not inject CO2, keep light moderate and photoperiod shorter. If you do use CO2 and bright lights, ensure consistent CO2 levels and a balanced fertilizer routine. Sudden changes in CO2 or fertilizer can invite algae blooms.
After Big Pruning or Re-Scapes
Heavy trimming reduces plant mass, so they use fewer nutrients for a while. Reduce the light period by an hour or two for a week after major trims. Resume normal light once plants recover and new growth appears.
A Simple Step-by-Step Recovery Plan
Day 1: Test and Tidy
Test ammonia, nitrite, and nitrate. If ammonia or nitrite is above zero, feed very lightly and consider adding a bottled bacteria product to support the filter. Clean the glass, gently rinse filter sponges in tank water, and vacuum the substrate during a 30–50% water change. Set your light to 6–8 hours with a timer. Block sunlight if the tank is near a window.
Day 2–4: Choose a Quick Clear Method
Use a blackout or run a UV sterilizer if you have one. During blackout, keep the filter and air running, and avoid feeding more than a small pinch if necessary. If using UV, run it continuously until the water clears. If you choose a clarifier, follow instructions and swap or rinse mechanical media after clumps collect.
Day 5–7: Stabilize and Prevent
Once the water clears, do another 30–40% water change to remove dead algae and dissolved nutrients. Add fast-growing plants if you do not have them already. Set your weekly routine: consistent feeding, weekly water change with gravel vacuuming, and gentle filter maintenance. Monitor nitrate and phosphate for a couple of weeks to ensure levels stay reasonable.
Common Mistakes to Avoid
Leaving Lights On Too Long
Running lights 10–14 hours a day in a low-tech tank is a common trigger. Use a timer and keep it to 6–8 hours.
Overfeeding and Skipping Maintenance
Extra food becomes waste, and skipped water changes let nutrients rise. A simple weekly routine prevents most algae issues.
Turning Off the Filter at Night
Filters should run 24/7. Turning them off can starve beneficial bacteria of oxygen and cause mini-cycles and algae blooms.
Cleaning Filter Media in Tap Water
Chlorine and chloramine in tap water kill the bacteria you need. Always rinse media in water you removed from the tank.
Relying Only on Chemicals
Algaecides or clarifiers may clear the water briefly, but the algae will come back if light and nutrients are not managed. Use chemicals sparingly and focus on balance.
Frequently Asked Questions
Are my fish in danger right now?
Usually, no. Most fish tolerate green water. Watch for gasping at the surface at night, which signals low oxygen. Keep good surface agitation and consider adding an air stone during a bloom.
Will a UV sterilizer harm my fish or plants?
No. The UV light stays inside the unit and treats only the water that passes through. It is safe when used as directed and is one of the most reliable solutions for green water.
How long does it take to clear?
With a blackout, expect 2–4 days. With UV, you may see results within 24–72 hours. Without these methods, clearing can take weeks of strict maintenance and light control.
Do water changes make it worse?
Water changes help if your tap water is low in nutrients. If your tap water has high nitrate or phosphate, a big change can fuel algae. In that case, use a phosphate remover, consider partial RO water, or do smaller, more frequent changes.
What about algae-eating fish or shrimp?
Amano shrimp, otocinclus, and some snails help with surface algae, but they cannot filter free-floating algae from the water. They are helpful in a balanced tank but not a direct cure for green water.
Is green water the same as a bacterial bloom?
No. Bacterial blooms usually look milky white or gray, not green. Both come from imbalance, but green means algae and milky means bacteria. The fixes overlap, but UV and blackout target green water directly.
Real-World Examples and Tips
New 20-Gallon Tank Near a Window
The tank looks fine for two weeks, then turns green. The light runs 10 hours, and afternoon sun hits the glass. The fix is to move the tank or block the sunlight, reduce light to 6–7 hours, do a blackout for 3 days, then keep weekly water changes and moderate feeding. The water stays clear once sunlight and feeding are controlled.
Heavily Stocked Community Tank
Tons of fish in a 29-gallon tank create constant waste. Nitrate reads 80 ppm, and water is green. The fix is to cut feeding in half, increase water changes to 40% weekly with thorough gravel vacuuming, clean filter sponges in tank water, and add a UV sterilizer for fast clearing. Long-term, reduce stocking or upgrade filtration.
Planted Tank With Strong Light, No CO2
A bright LED runs 9–10 hours over a lightly planted tank. Plants struggle and algae bloom. The fix is to reduce light to 6–7 hours, dim the light if possible, add fast-growing stems and floating plants, and fertilize lightly once plants start growing. Once plants thrive, algae fades.
Simple Checklist You Can Follow
Right Now
Test ammonia, nitrite, and nitrate. Cut your light to 6–8 hours and block sunlight. Feed less. Clean the glass and do a 30–50% water change with gravel vacuuming. Rinse filter media in tank water. Choose a quick fix like blackout or UV for faster results.
This Week
Do another medium water change after the bloom clears to remove dead algae. Consider adding fast-growing plants or floating plants. If tap water has high phosphate, add phosphate remover to your filter. Keep the light on a timer for consistency.
Every Week
Change 30–50% water, vacuum the substrate, and keep the filter flowing well. Feed carefully and watch parameters. When the routine is steady, algae problems usually stay away.
When to Seek Extra Help
If you deal with repeated blooms even after controlling light and nutrients, test your tap water for phosphate and nitrate. Consider mixing in RO water if levels are high. In very stubborn cases, a properly sized UV sterilizer is often the easiest long-term solution. If fish are stressed or gasping, add more surface agitation and review stocking levels and water quality immediately.
Conclusion
Green water is a clear sign that algae have found too much light and too many nutrients in your aquarium. It is frustrating to look at, but it is not a disaster and it is very fixable. Use a quick clear method such as a 2–4 day blackout or a UV sterilizer to restore clarity, then prevent a repeat by reducing light, feeding less, cleaning regularly, and keeping your filter healthy. Add fast-growing plants to compete with algae, and manage phosphate and nitrate if they are high. With a simple weekly routine and a balanced setup, your water will stay crystal clear and your fish will be happier and healthier.