Do yourself a flavor and buy a syringe. You can get one at Electronic Cigarette Kits, Eliquid by Awesome Vapor or at the local drugstore. You want one that has a large veterinary style needle that will pass think juice. It only has to hold a few milliliters but you want to be able to suck up some water to wash it out after filling a cart or tank. It's a lot easier and cleaner than popping off the bottom of the tank and putting it back on.
I keep my syringe on the window ledge behind the kitchen sink. When I fill a tank, I just pop it out of the atty, draw up about 1 ml of juice, flip the tank with the hole up and fill. Then you pop it back on and vape. I always draw up some water into the syringe (about half full), shake it a bit, and squirt it into the sink. That just keeps the syringe clean. It only takes about a minute to draw, fill, and clean the syringe. If you fill several tanks at one time, you are good for the day in one operation.
Good idea, thank you! That will be my next purchase. Those little caps are really hard to pop off.