i have just about reached my wits end with this specific problem, and it's the only problem of all of them that i have not been able to conquer: green water.
seachem clarity, which i tried as a last ditch effort, curtails the problem completely, but within a day it is back again. i tried a 3 day blackout, but all that really managed to achieve was practically killing off my ludwigia super red and making my s. repens really leggy.
my tank is an ADA 60-H (21g?) ADA aquasky moon light, and an oase biomaster 250
i run purigen and matrix, one coarse foam pads, and swapped out another coarse foam pad for fine filter floss.
stocked with 7 neons, 6 crystal shrimp, 3 otos and a handful of ramshorn snails.
i'm running CO2, and have had photoperiod ranging from 4-8 hours.
ammonia/nitrite 0, nitrates <5-10, phosphate 0, ph 6.4, temp around 22c
i am a habitual cleaner, i probably do 2x 40-50% wc's every week, sometimes more if i want to get in and trim plants, and simply because i enjoy it.
the filter, the pipes, outlets and inlets are all spotless, sand is white, and there's no evidence of any other algae anywhere else.
the plants are all (surprisingly) very healthy, they grow rapidly and show no signs of deficiencies despite me not dosing my tank at all since this problem showed up.