Eyebeatbadgers,
I can’t answer your question with certainty related to a higher output Gen-X pump, because I couldn’t find any pump output curves for them online. It is hard to find that stuff for smaller pumps of any brand. They usually do at least tell you the maximum head capacity, but I couldn’t find that either after searching for a few minutes.
The issue is that a larger needle pump might be able to flow more water when you are using it in a circulation-only type of setup (like with a powerhead in the aquarium), but still not be able to flow that increased water volume against any more head resistance than your smaller pump can, so you might still have the exact same issue with the pump backflowing. I would say that it is actually probable now that the first pump of the same brand didn’t work. With most smaller pumps, all that really changes as the pump capacities go up is the size of all of the parts, so the weak pressure capability of the smaller pump will most likely still be there with the bigger one.
With the setup you have now, the canister is having to push its’ output water through the needle wheel pump when the needle wheel is off at night, which probably does cut the canister’s output flow a little bit. You could eliminate this by experimenting with a setup where you take the exact same plumbing setup you have now, and you just add a short second canister output line that T’s off the existing canister output line right before the needle pump, bypasses the needle pump, and then T’s right back into the existing canister output line just after the needle pump.
With that setup, there would be no pressure difference between the intake and output of the needle wheel itself at any time, both pumps will still share the same plumbing into and out of the aquarium, and since the needle wheel is on a secondary parallel leg of the canister output it won’t restrict the canister’s flow at all when the needle pump is on or off.
That would be something worth trying, and based on your pictures it looks like you already have all the parts you need to try it. Worst case scenario would be that the total flow out of your spraybar goes back to what it was when it was just the canister filter during the day, and at night your flow is increased a little from the canister not needing to pump water through the stopped needle pump.
You might however find a worthwhile benefit that would be caused by a weird interaction with the flows through the canister output line and needle wheel line when both pumps are running. The interaction would seem very counter intuitive, but it would very likely actually help chop up and dissolve your CO2 more fully so you would have less micro-bubbles of CO2 in the tank. I am not going to tell you what would happen or why though. If you do find there are less CO2 bubbles in the tank, you can learn a lot by trying to figure out why.
A clue to what might happen can be found by watching the CO2 mist in the lines. If you can see through the pipes when it is all up and running, look at the water in the lines and see if you notice anything strange about the direction some of the CO2 bubbles are traveling. Another clue is that it will only happen if your canister is flowing less total water then your needle wheel is pumping. If everything looks/flows just how you would expect, that just means that your canister has a higher flow capacity than the needle pump with your particular setup.
Have a good one, Jeremy