Follow the mfg's instructions. Most mixes (including IO, Oceanic and Reef Crystals (I use all 3)) do say it can be used immediatly after mixing. However common sense and experience (And the mfgs instructions) dictates that in order for the new mix to stabilize you will want to let it mix for about 24 hours. This gives everything a chance to dissolve and PH, buffers, etc to become stable. In small quantities I have found it doesnt hurt to mix and then wc
(ex I change 5g in my 75g, I have no negative affects from mixing and then wc
right away) In larger wc
's you should always match ph, sg
and temp to avoid sending your critters into a state of shock and possibly death.