Yes, Biosys can be mixed with any type of water, but it performs best with dechlorinated water since chlorine can harm beneficial microbes. You can use rain water, or RO filtered water as this is chlorine free. If using tap water you can gas-off the free chlorine by letting it stand for 24-48 hours, or use an air pump and air stone for 1-2 hours, but this doesn't remove chloramine that is often present. Adding a 1-2 drops of Ecothrive Neutralise per litre will dechlorinate tap water instantly.