Replaced gas tank. Vent tube not working

I recently replaced the gas tank with a used tank. Vent tubes identical. Have tried repositioning tube, etc, but it will not allow me to put gas in at more than a trickle. I have had the filler pipe off, vent tube out, and there is no blockage anywhere. If I move the vent tube up or down slightly, it makes a difference, but still not enough to allow decent gas flow from the pump. It takes me about 20 minutes to fill the tank. The problem is not getting gas to the engine, it is putting gas in the tank. The air cannot come out of the tank fast enough, and the gas keeps backing up, and running out the fill pipe. I have been told that it is probably an after-market tank, and I won't be able to do anything about it. I can't accept this. Ther must be a way to put gas in tank