The entire question of tap vs. RO/DI has been greatly debated among many on this site. Basically, it's better for the LONG RUN to use the absolute purest water you can get, RO/DI. Yes, tap water may be ok, just depends on what is in your water. The most important thing is to get rid of all chlorine and chloramine, as they are toxic to fish. Stress coat will do that. The Ro/DI water also is free of nitrates, trace metals, ets. These things can cause big headaches down the line, especially in a reef tank.