Some times coils do pop, its normal/just that coil head.
As for the sub tank needing at least 20w... I run mine at 15w - 18w, and the tank performs just fine, I prefer the lower power as the juice I have in it is a vanilla/red Astaire mix that is much creamier at lower wattages. naturally you need a mod with Buck to get as low as 15w with a .5ohm coil, but my DNA40 and SX350 and Sigelei 100w and IStick30, all do, and is why I avoid mods that don't buck... DNA30, istick20, sigelei mini etc...
As for the sub tank needing at least 20w... I run mine at 15w - 18w, and the tank performs just fine, I prefer the lower power as the juice I have in it is a vanilla/red Astaire mix that is much creamier at lower wattages. naturally you need a mod with Buck to get as low as 15w with a .5ohm coil, but my DNA40 and SX350 and Sigelei 100w and IStick30, all do, and is why I avoid mods that don't buck... DNA30, istick20, sigelei mini etc...