FM,

One more reason why you might need to do a 100% water change is when your NH4+/NH3 or NO2 levels are off the chart. NH4+/NH3 usually shows itself as very fine bubbles sticking to the glass at the water surface. If NO2- levels are high, your fish will usually be gasping at the surface. The best way to tell would be to invest in NH3/NH4 and NO2 test kits. They last a long time, and you can use it for any of your fish tanks.