I like your res idea and I think I can picture what you have in mind...here is my humble opinion:
Consider leaving out the tee, using an elbow or other means to attach the pump to the bottom and letting the UV clarifier be your top or lid to the res. You would have to have a water inlet so you could use your tee at the top or just tap the tube for hose barb(s). You might want to attach your pump to the res with some short length of hose to limit resonance / vibration.
That way you could buy it (the UV5) later and not even have to drain your water to put it in.
I have also noted the comment about string algae. It may be that this stuff is immobile and never gets washed through the system to see the UV light. That is the shortcoming of this approach to keeping a clean system.
I don't think that we need "beneficial bacteria" in our systems b/c we don't have anything we want digested.