I always detach the regulator off the disposable tanks.
Since I haven't had a way to assess the quantity of oxygen added to my wort, I never have worried about the amount. But as James mentions above, 1.5 L/min for 5 min might be a lot. I'm betting there is some sort of stoichiometric calculation as to the appropriate amount of oxygen to add to a mass of wort to achieve a certain dissolved oxygen content in the wort. I know I can dig it out of my wastewater engineering texts, but has anyone already done that calculation?
I also have a little 10 lb oxygen tank, but I've been waiting to use up the 2 disposable tanks I had already purchased. It should be much cheaper as snowtiger (why don't you add your name to your signature line?) points out.