Green water is caused by algae and similar microorganisms growing in your water. It often happens in new plant tanks when there is an imbalance between the light, the nutrients and the CO2.
If you are not running a planted tank, the simplest solution is to blackout your tank for a couple of weeks and do some 50% water changes weekly.
If you are running a plant tank it is more difficult to control greenwater. You can try a 1-week blackout with no lights on in the tank and covering the tank glass with a blanket or dark paper. You can try frequent water changes. Neither of these methods are likely to work.
If you can borrow a diatom filter, run the diatom filter to clear the green water. You will need to repeat this weekly or so until the underlying cause of the green water (excess nutrients in the water) is corrected.
The simplest and easiest way to clear green water is with a UV sterilizer. You do not need a high wattage. Run a UV sterilizer and you water will clear within several days. Keep the UV sterilizer running for at least 2 weeks after the green water has cleared. After that you can turn off the UV sterilizer and most likely the problem will not come back.
UV sterilizers are expensive and they only need to be used once or twice. I have 2 UV sterilizers both sitting in a box in my basement waiting for the need to arise. If you can borrow a UV sterilizer that would be best, it is a waste of money to buy one that you will only need for a brief time.
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