lookin great in there. Love the density, don't see many growers will to actually use the real estate available to em these days. No light hits the ground is also a rule I try to adhere to.
only time you need to be concerned about salt build up is if you're letting the media dry back too much, and feeding excessively. If you're dialed in, then little to no runoff is probably the absolute most efficient way to grow. Even commercial growers (not just cannabis, in general) in rockwool don't have runoff until the fourth or fifth fertigation event in a day, and even then it's pretty minimal.
