I've been having the same "looseness" problem with my tank too. It's not leaking as such, but I did dump 5ml of juice on my desk when the bottom of the tank slipped off when I tried to remove a drip tip. I definitely don't think the O-rings are big enough to keep a tight fit. I did however figure out one trick that helps some. Make sure that you have your tank completely clean and dry, when putting it back together. If there's the slightest juice on the tank walls it acts like lubricant, which is why I think my tank came apart. I cleaned and dried it thoroughly and put it back together and it seems a bit snugger now, but still not super tight. I also noticed that these o-rings seem to be very slick rather than rubbery feeling...that may be contributing to it too. It just needs a bit of sticky to give it traction to hold to the tank. Just my thoughts. If Bruce has any better o-rings I'm game to try them.