Plants were started from seed, the plant pictured is the purple PRK from a previous pic, btw. I started the seeds in 1" rockwool cubes, once they sprouted up I put the rockwool cube into the net cup with hydroton, I keep it kinda wet with plain water until I see roots creeping through the hydroton. Once I see roots, it goes into the reservoir.
Initially there is no air gap between the roots and solution with Kratky, the reservoir gets filled so that the bottom half of the net cup is submerged. At this point there is sufficient dissolved oxygen in the water to sustain a small plant. As the plant drinks up nutrient solution and grows roots and shoots, the water level drops creating the air gap, as the water level drops the roots follow it down. At this point, my only concern is maintaining the air gap when I add nutrient solution, so I will only fill it up to about 2/3 full leaving roots out of solution to uptake oxygen.
One last thing...to combat algae, I add pond-zyme to the reservoir. You could also black out the lids to keep light out.
I hope that clarifies things. Let me know if you have any other questions...I'm no expert, but I've learned a lot as I go and think it's an interesting way to grow stuff.
View attachment 4218767