First of all, you shouldn't change any water in the tank until you are completely cycled: ammonia 0ppm, nitrite 0ppm. Your tank is not cycled until all the cloudiness goes away. If you change any water before this happens all your doing is taking out beneficial bacteria which took three weeks to build up. After the tank has cycled "completely" and you have a decent amount of bioload, then you start changing water in it, not before.
First of all, I'm talking about cycling a new tank for the first time and to accomplish that your water must be free of all ammonia and nitrites. At this point the water will have cleared up and be "cycled". At this stage you can add a few fish depending on the size of the tank. If you add too many fish too rapidly the beneficial bacteria can't keep up with the bioload and you end up with too high of an ammonia content in the water, even though the tank had been cycled before.My understanding has always been that the bacteria doesn't live so much in the 'water column' but in the filter, on the substrate and decor etc. My 13gal tank is now fully cycled after the nitrites spiked, and I had to change the water on it twice a day to keep the fish alive and the nitrites under .25ppm. If I had left it to get cloudy...I couldn't imagine the numbers I'd have gotten. As it was even changing it twice a day the nitrites would get to 1ppm, with crystal clear water.
Maybe one of the more experienced (I've only had 3 tanks total) aquarists/moderators could chime in here and set us on the right path!
Cris
First of all, I'm talking about cycling a new tank for the first time and to accomplish that your water must be free of all ammonia and nitrites. At this point the water will have cleared up and be "cycled". At this stage you can add a few fish depending on the size of the tank. If you add too many fish too rapidly the beneficial bacteria can't keep up with the bioload and you end up with too high of an ammonia content in the water, even though the tank had been cycled before.
There is bacteria in the water column and everywhere inside the tank and anytime you change too much water or vacuum too much gravel you are removing too much bacteria from the tank.