I have really green water in my tank. I cut down the light they get, cut down feeding and used green away. Nothing has worked. I had the tank for 4 years now and this is the first time the water has been this bad

How often do you change the water, when you have an algae bloom you may need to change as much as 50% of the water several times before it comes under control, cut back on feeding if they are not eating the food quite quickly and as you said cut the sunlight a bit too.

Normally once a week. I have a 120l tank and 1 bristle nose , 8 black widows, 3 neon and 4 lemon tetras . I'm not refilling tank as once they die I'm not going to keep doing it. So my tank isn't over stocked. I have heard of blacking out the tank for 3 to 4 days but some Ppl are for and others against. Thanks for reply

Could it be temperature related ? we have never run a heated tank but I seem to recall it's pretty critical to keep the temperature right and filtration may be needing some help.

when i kept tropical fish over 30 years ago, the green was to much sun light entering the tank. the same as algae in a pond..