you should really use RO
water if you can. many LFS
will sell it by the gallon. If you don't have access to that then you should treat your water before adding it to the tank as you don't want anything from your tap water to settle and remain in your substrate. So, use RO
water if you can, if not, age and treat the water outside the tank, then add it to the tank. Note, don't fill it up all the way as you'll be adding LR
that will likely take up a large amount of the tank's volume. I wouldn't fill it past 2/3 the way up. Bring the water up to temperature and add your PHs. add some salt and let it mix for a while before testing. once you've got the water up to SG
1.023 let it mix over night and test again to be sure before moving forward with your cycle.