raekje

I have set up a fish tank with 2 gold fish and one black fish, this has been placed in our study where our business is operated from. The last 2 months, until recently, one gold fish has had a white fungus growing on it but he did not die from it, Now every week the water turns Green with nothing put in the tank to cause it to change colour, it is filtered. The tank goes murky green within a week of being cleaned. I would be very greatful if some one could answer why the tank is doing this. It has only just started to change colour in the last weeks and the fish is getting better. Please help.