Since I just started doing this, I haven't had to fill a jar, yet. My understanding is the nutrient mix in the jar can last the whole life of the plant (lettuce in my case). As the lettuce grows, it begins using up the nutrient mix. Some of the roots stay in the water as it recedes, and some are exposed to the air in the jar which is important because the roots need to be exposed to oxygen, too. If the nutrient level gets low, you can always add more, but do not cover all the roots (fill up to the top again) or you will starve the roots of oxygen and the plant will die.
From what I have read, the jar should be cleaned and fresh solution used for each new planting.
When I started, I filled the jar right up to the ridge line below the top (see video in the first post of this thread). When I put in the net cup with the clay pellets, the water rose to the level of the top clay pellets, but did not cover them. A couple of the seeds were actually floating in a bit of water, some landed on the moist pellets. Both sprouted. Remember, the water level will begin to go down somewhat just from evaporation.
I started with jars as I wanted to just try this method out before committing to it. I imagine I will move, at some point, to larger tubs or buckets where I can grow more plants in a single solution reservoir.
The ideal temperature for this type of hydroponics is 68-72. That is why it is ideal for doing indoors. Lower temperatures will not hurt the plants, but the growth will slow down as the temperature drops.
I have 3 shelves on my rack. If I remember correctly, I spaced them 20, 24 and 28 inches. However, I also hung my lights using shoestrings and those little toggle spring clasps, so I can easily raise or lower my lights as needed.
Toggle spring clasps
