As long as your fish are not dying, a green tank means a healthy tank but it is not attractive to look at! Too much direct light could be causing this, as someone mentioned get some bristlenose cat fish or even sucking catfish. 25% change of water each week. Try to steer clear of the 'algae eating' potions the pet shops will try to sell you, the less you need to add in the way of medications, the better. If you have a heavily planted tank and not enough filtration this could also be causing your green tank. Cellophane filters the sun, so you could get some of that and wrap it around your tank for a week or 2 and see if that makes a difference. My tank is not in any direct sunlight but still gets a green tinge to it. Good luck