Very good questions you have, ok the botton lines between the tubs are 32mm piping that connects them all together so the water level is always the same in each tub and resivor are are even, so the 19mm top tubing is just used to circulate the water to the tubs from the resivor to keep it fresh, and yes there is always water in them. The setup actually has another tub on top of tub 1 and 2 that has rockwool slabs in then and the bottom of those tubs is like a mesh so the roots and water can come out the bottom into the bottom tubs, i have another
pump with another 19mm tube running to the top tubs that had little drip tubes feeding the plants.