Problem with that idea is the small screen will get clogged and your plants can get root bound. There is nothing wrong with growrocks or lava rock dude if you want to grow hydro then do it the right way or you may not get any harvest.

You are using additives as a grow medium and that is very hard to do but you figured that out already.

You may want to finish them off in a dirt mix then but it's up to you.