If your just starting up a tank, and want to mix up saltwater to fill a tank so yu can starat to cycle your live rock, just dump it in the tank. Its not a big deal. Mix it up in 5 gal or larger batches and our it into the tank. Add a bit more water to bucket etc to make sure its all out and use power head in tank to ciruclate it. Water to start a tank cycling is not all that inportant in so far as needing to let it set for 24 hours prior to use. There is no way your PH and other paramters are gonna be anywhere dead on no matter how you do it. Worry about that after its fully cycled... Just get your salinity in the range you need it, and once rock and sand is in place and powerheads running, then make adjustments to bring up or take down the salinity to the level your wanting to use once its setup and cycled.