I love their tanks, as a matter of fact, I'm waiting for my order of two more of the 3ml polypro tanks. I also have one of their pyrex tanks and it has been my daily companion, that and the 3ml polypro. It took me a few refills to get the hang of these tanks, but now I just do it in 30 seconds.
The beauty of the polypros is that you don't need to disassemble the tank to refill them, just use a syringe and refill from the screw hole. When I change a carto, I assemble the unit empty and then fill from the hole on the side.
Sometimes, if I flood the carto somehow, which is not very often, I just unscrew the tank, clean the battery contact and blow out the extra juice.
You can tell if the grommet is in right if you see the o-ring sitting in the tank evenly. If there is any part of it that is higher I just gently push in until it's even.
Hope this helps, I love their products.