Clay Pebbles, Clay Balls or Leca - the only thing you need for your semi hydro set up.
 
The balls of clay wick up water, meaning a little pool of water at the bottom of a pot made for semi hydro can actually hydrate all the way to the top!
 
The balls also provide chambers of oxygen for roots to grow super strong, which will lead to amazing foliage.
 
Alternatively, you can add clay balls into soil for extra water retention in your mixture.