In preparation for setting up a 20gallon cube, and being limited in space for buckets, I'm wondering if it's okay to make the salt water directly in the tank? Ie. I would fill it with RODI water, then add salt and a powerhead to mix the salt in, and then put substrate in later...or should put the substrate in first fill with RODI water, and then add salt.....or is this a bad idea, and if so why?