you then apply water tight glue to the bottom of the bottle top and stick it on to the top of the tank
Step 2: The drill
you then drill straight through the bottle top and the tank being sure to leave enough space at the sides so the glue can keep the cap attached to the tank.
Step 3: Use it
all you need to do is fill up a bottle that fits the cap with water, and screww it on to the cap. the water will fill up the tank giving you easy refills. you could even carry a few bottles around so that you could refill without going to a tap.