Personaly i would rebuild the whole thing and use foxfish's design. 🙂 And this one, it still needs a valve where the red cross is to regulate flow over the diffuser. And 45° T's instead.
View attachment 112285
Since you are using pieces of flex tube to connect the diffuser to the PVC tubing.. Than take a look at these valves.
https://www.banggood.com/Aquarium-F...w-Control-Switch-1216mm-1622mm-p-1033125.html
Is this what you feel would be best then? As I think I’ll order the parts over the next couple days. Again, thanks for your help.