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!