1. How deep must the airstone be in the water to get maximum oxygen? 2. How can you determine if the plants are getting enough oxygen? Thank you. Koos van Rooyen.
Deep Water Culture
Certified GKH Gardening Expert
The airstone should be placed on the floor of the tank. This site has illustrations of a few different setups:
The simplest answer to your second question is: You can tell if they look happy. If the plants aren't getting enough oxygen, the roots will start to smother and you'll notice a rapid decline in health.
This article will tell you more about Deep Water Culture: https://www.gardeningknowhow.com/special/containers/deep-water-culture-for-plants.htm