Thanks for your responses, the problem resembles blanket weed that is found on ponds. In the water it is like cotton wool floating on the surface and around weed. When removed it is considerably reduced in volume and appears like green string? is there a good website where I can identify? The tank is away from any natural light, but we do have the tank's light on from about 7:30am - 10:30pm. We have also noticed that the water in the tank is quite warm to the touch - tepid rather than room temp.