UV can be expensive for some folks and the oxidizing of nutrients is still a mystery...why not do the 3-5 day blackout method as suggested, aerate the tank and make sure that no light get in there...no feeding during the period is best as it minimize risk of light getting in there. (Don't worry, your fish would be fine for the short period.) Next, you would need to correct the conditions that cause the greenwater. Why not give us the details of your tank and fert regime for us to help you.
Okay, pH and KH test kits help you determine you current CO2 level using the table formulated for this purpose.
Nitrate and PO4 are the biggest things to watch for nutrients as kits ain't available for testing of each and every micro which can be a PITA. When all nutrients are in place, you would be a drop in N and P..if you don't, you would need some detective work to find out what is limiting the growth.
Plant Physiology by Taiz and Zeiger
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