Custom fuel tank, vent question

I just ordered and received two round fuel tanks, 5" round by 10" long. They are going on a homemade trike I've built. The tanks came with non vented caps, a fuel out petcock on one end and a vent opening on the bottom of the tanks on the opposite end. I have the vent tube and fittings. I understand that the vent tube needs to come from the bottom of the tank and rise up above the fuel line on the outside of the tank, where the vent tube will remain uncapped and open. On the inside of the tank, I'm guessing the vent tube wound need to extend up into the tank high enough so that it's always above the fuel line?? If so, what keeps the tube from shifting and possible dropping below the fuel line? Doesn't the vent tube need to remain clear? The vent tube is just a propylene tube.