I wouldn't use tap water. IF your tap doesn't have a lot of "gunk" in it, you may be able to use it. I would go distiller just in case. By gunk I mean dissolved solids...I have read people using it but they have teste the tap with TDS meters, etc. so I would just go get a $2 jug of distilled...IMO. Also, you need to let the solutions sit for 24 hours before using them, so dosing tomorrow morning isn't the best option.