The first pump is for just keeping the water moving if you don’t have a water chiller - I highly suggest a chiller, as it makes the water easier to oxygenate and makes it harder for bad nasties to grow. I personally set my water pump in its own bucket that had no plants in it - I didn’t want roots growing into my pump (and even then they still tried! Traveled down my 8” of pvc and aimed right for the water pump). But that’s also not necessary.
Oooo I’m the wrong person to ask. I never changed mine. I just let it deplete almost totally once every month or so, and refilled the tote with fresh water once that happened. I ended up with some minor issues due to the Recharge and Molasses gumming stuff up - if you’re gonna use either of those, I suggest doing a proper clean at least between every grow. I am very lazy so I did not bother.
Nope. Rockwool cubes. Those are my go-to. Others like to use Root Riot/peat root plugs. I found those tended to shrink as they dry out and could cause damage to my stalks if I wasn’t careful to keep them hydrated.