Bill, the reason to avoid big water changes when you are doing a no-CO2 tank is to avoid introducing CO2 with the water change, not other nutrients. Tap water usually has some CO2 dissolved in it, so adding half a tank of tap water adds considerable CO2. Then the CO2 dissipates and is used by the plants, bringing the tank back to under 3 ppm of CO2. Those peaks of CO2 followed by very low CO2 can trigger algae blooms.