i think a 3" return tube would be too big and you wouldn't get the right pressure your looking for.. honestly a 1" should do the trick, also if the tank on the right were at a higher elevation gravity would give you some help as well...
hmm yeah.. w.o n overflow that would be an issue...
I saw you holding what looked like a second tubing in the video.. have you tried using 2 of them yet? 2 at that size i think would be better than just 1 big one..
other question... why such an elaborate filtration for that size of tank? lol I'm just curious