Tap water is probably fine, but if you live in the city, your water may contain high levels of chlorine and chloramine. If it is chlorine, then you can allow it to dissipate by simply letting the water sit out uncovered over night. Chloramine requires neutralization to remove it. I prefer a product by SeaChem called Prime. It is available at most aquarium supply stores. I buy "Pond Prime" in two liter jugs from Drs. Foster and Smith. It is the same stuff, just extra concentrated, and a two liter jug lasts a good long while, even with 19 fish tanks ranging from 150 gallons on down