Fish tank cleanliness help!?

My daughter has a moor and 3 gold fish in a 10 gallon tank. We noticed that the water gets cloudy and yellowish about 2 weeks. How do we keep the water sparkling clear? We replace the filter ever 2 weeks, and we always put the necessary additives with the water. How often are we supposed to change the water? We do it every 3 weeks. Should we do it more often?