To fill, turn the tank upside down and remove the threaded end cap.  Fill the outer portion of the tank (avoid getting e-liquid into the center airflow tube) and then screw the end cap back onto the tank tightly.  You can now turn it rightside up and check for leaks before using.

Gurgling or Leaking can occur if liquid gets into the center airflow tube.  To clear this liquid and improve performance remove the tank from the battery and blow through the mouthpiece into a paper towel.  The excess liquid will come out of the threaded end of the device.  If this happens often you may need to check that all connections are screwed down tightly or replace the head and wick assembly with a new one.

The head and wick assembly can sometimes loosen itself when the threaed end cap is unscrewed.  Always check that it is pushed into the bottom cap tightly tightly before screwing the cap back on.  Not doing so could result in a leaking or non-working device.

You should never run the tank completely dry.  Doing so could result in a burnt or damaged wick and can cause leaking.  Refill the tank when the liquid level reaches approximately 1/3 full

The lifespan of replacement heads can vary quite a bit, but following the above instructions will maximize the lifespan of the coil. Replacement Heads for this product can be found here