Is my fishtank overcrowded?

I have 6 goldfish, 2 are about 6 inches long, 2 about 4 inches and 2 about 3 inches (including fins). I also have 2 catfish about 2 inches long. The tank is 40L in capacity. Is it overcrowded? We have 2 filters and 2 air pumps but the water is very green. We have plastic plants for shade and ornaments for the fish to play in and around. I think there may be too much direct sunlight but there's nowhere to put it that would be any better. Please help! Thanks.