I guess I should have continued to draw the next part of the installation, as there is a huge misunderstanding of the actual concept and functioning.
Look, it is much simpler:
1. Firstly I was using (and still recommend) a 5L or 1-1/4 gallon reservoir as mentioned earlier - they are cheaper and use less power. But most importantly, they do not turn on by water pressure or flow, but in fact strictly thermostatically.
2. I called the secondary reservoir so because of it's relation to the 5L tank of the heater.
3. The circuit (actually not shown integrally) here
is a separate and closed circuit,
no nutrient actually flows through it, but tap water. The main hose only (connected to the heater's in- and outlet - as in yellow and blue) gets dropped (lays) inside one or several nutrient reservoirs and heats it indirectly. That is why I said that a thinner hose should be used inside the reservoir(s). In fact to better conduct and transmit the heat from the hose to the nutrient solution.
4. The "mechanics" of this concept are actually unbelievably simple and one tends to over-think it by anticipating possible errors. Just trust me, this system is thought trough very well and is working perfectly fine if understood and used in the right way.
5. One more hint, if the heater unit (secondary tank and
pump also) are positioned at the same level (hight) of the outlet and inlet level (both are suposed to be at the same hight then) of the hose entering (and exiting) the the reservoir(s), it doens't need to be very powerfull. There are two options: running the pump by cycles or running it permanently. By cycle will save some energy and permanently will assure very stable temperatures of the nutrient solution.