They're just two caps pressure fit on a glass tube with o-rings. Nothing fancy. I would hope you don't need instructions to disassemble them.
My first choice would be a fingernail. Even short ones work fine. If I couldn't use a fingernail, I'd try a different fingernail.
Except for one of the smaller diameters, the glass is rather thick & very resilient. Even if you dropped it, it'd have a pretty decent chance of surviving.
I pull the caps on mine (I have 3 19's & 2 22's) for deep cleaning every so often. It's not that big of a deal. When you first get them, they can be a little stubborn to get off, but it's still not that difficult. The o-rings will break in & soften up a little after you remove them a couple times... not enough to make falling off or having a carto push them out an issue, but a touch easier to pry off.
DJ, I use the cartomizer. Push it down into the tank and tilt it to the side then use it to push the top cap off, sometimes I push the cap off a little on one side then tilt the cartomizer to the other side and push there. Once the top cap is off slide it out some and use it to leverage the bottom cap side to side, just dont lever over with any kind of huge pressure and it will wiggle out. If it is stubborn then pull the cartomizer out and use it from the other end through the tank to push again from the inside like you did with the top cap.
I wouldnt use any kind of metal slid in between the cap and glass tube. If you note the design of our tanks the metal can never touch the glass because of the washer and oring. Metal is like kryptonite to glass and the impetus for our reasoning in the design.