Conecting 2 NFT's with control tank

[Toleman]

Hi im new here and started growing hydroponicly around 3 months ago.

As an intro I have done what all the newbies do and got the smallest system. So far my chilli plants are doing well. Im getting lots of pods and im thinking about expanding next season.

Anyway I have a small nft which fits 2 plants and Im thinking about buying a bigger one for next year. The set up is in my greenhouse and the plan is to connect the new bigger and current together along with a control tank.
The reason for this is my current systems tank is 20 litres and when its hot the plants are using 4 litres and I would like to be able to leave them a bit longer. They are also depleting around 5ml of each grow, bloom, micro (GH)
My greenhouse is pretty small so a stand alone ressy will be easier to tinker with.

Anyway here is a basic paint sketch and im wondering the best waterproof fitting to link the return to the control tank. (shown in red)