And if you are talking about -T type
tanks (a clear container for liquid use with an atomizer with a spike to puncture the tank), many prefer to pre-puncture the tank on the atomizer then remove it from the atomizer and use a syringe to fill through the hole.
If you mean a cartomizer (holds liquid and also screws right on the battery with no atomizer) - some cartos do not have removable endcaps. Buy decent cartos like Boge 510 (slim batteries use the 3 ohm ones, fat batteries can use 2 or 3 ohm)
If you mean cartridge (black plastic with stuffing and pushes onto a normal atomizer), there is no cap. You remove it from the atomizer and drip
juice into the stuffing.