Thanks Smurfy, but when you say clear in time, by the ime I do another change 4 weeks later it gets so cloudy/ milky you can just about see the back of the tank.
4 weeks is a long time between water changes. Are you vacuuming the bottom/substrate when you do a water change? And how much volume are you changing? I think that if it gets that bad, then you probably need to do more frequent water changes and/or larger volumes.
Can you describe you setup for us? Tank size, filtration, tank occupants, type of substrate.