If you really like those GS Air 2 
tanks that are used in the iStick Basic and want to use one on your Zmax you can get some 1.5ohm coils for it like these 
Eleaf GS Air Coils - Eleaf Coils - Eleaf       - Tanks & Coils
Also, heres a bigger version of that tank 
GS Air 2 16.5mm  - Eleaf Tanks - Eleaf       - Tanks & Coils
The coils that come with it are .75ohm which is a "Sub-Ohm" coil that most tube mods wont fire because the resistance is too low, which is why I suggested the 1.5ohm coils above.
A LOT of people really love the original GS Air tanks as-well, mainly the GS Air M along with the Original Coils I linked! 
 
Edit* Not saying to run out and buy it, just a reference for the future or if you were wanting too already!