Shorten the light schedule.
Use a timer, block direct sun, and avoid leaving lights on all day. Do not black out a stressed tank without checking oxygen and livestock needs.
The Hidden ReefAquariums · Fish · Coral · Ponds
Water clarity guide
Green Water Bloom
Green water is usually free-floating algae, not dirt. The fix is to reduce light and nutrient fuel, improve filtration, and use UV only when the system and flow rate make sense.
First response
Do this before adding algaecide.
Stabilize the basics and remove the fuel source.
Test nitrate and phosphate.
Green water needs fuel. Overfeeding, dirty substrate, high fish load, and overdue filter service can all keep the bloom going.
Clean mechanical media gently.
Rinse pads or floss in removed tank water when appropriate. Do not replace all biological media during a water-quality problem.
Likely causes
Why the water turns green.
The answer is usually light plus nutrients, not one missing chemical.
Window or seasonal sun exposure
A tank can suddenly bloom when the sun angle changes, even if nothing else seems different.
Too much uneaten food
Extra food breaks down into nutrients. Feed smaller portions and remove what fish do not finish quickly.
Immature filtration
New tanks and newly cleaned filters can swing while bacteria and routine settle in.
Helpful departments
Where to shop next.
Pick supplies based on the cause you find.
Nitrate and phosphate tests
Use test results to decide whether the bloom is nutrient-driven.
Shop maintenanceUV sterilizers
Useful for free-floating algae when sized and plumbed correctly.
View UV clarifiersMedia, floss, and replacement pads
Fresh mechanical capture can help clear water while biological media stays protected.
View filtrationWhat to avoid
Do not dose algaecide blindly in planted tanks, shrimp tanks, ponds, or reef systems. Do not clean every filter part at once. Do not keep doing full blackouts without finding the light or nutrient source.
Compare with the algae guide