Hello simon,
I just read the reply from the tech, I will send you a private message with the reply in full because I'm not sure it would be ethical to post a private e-mail publicly without their permission first. But it seems to be a lack of dissolved oxygen in the
nutrient solution. In order to support the canopy, the root mass is gobbling up the oxygen faster than the air stones can dissolve it into the solution, and that a bigger reservoir with more circulation and aeration is the key.