I just got some 510 cartos from FSUSA. They were a little different than the others I've seen. I had to pop off the bottom cap first and there was another one inside it I had to pop off. when I got both of them off there was a little opening which I assumed was to put the liquid in. I was only able to get 6 or 7 drops in it until it started dripping out the bottom. Did I do this right? I mean its working but I was hoping to get more in there