I bet he is using well water with a lot of iron in it or a metal trash can. He should be using cal-mag but all answer were not given. Many variables. When I start with R/O water I know there is nothing and I control what goes back in it. My bro uses well water at about 140 ppm's and only needs a litle cal-mag to bring it up. Lucky him.

I have read up on some Africans growing hydro in old military metal containers and floating pots in them. They send the young ones out a couple times a day to whip the water with stick to get air into it. Works for them and they are feeding themselves in the desert.

I have used drip systems with no air stones only water splashing back down in rez and done fine.