The number before the / is the amount of ci (cubic inches) the tank is. The second number is the maximum pressure the air can be held at inside that tank. Therefore, a 68/4500 will hold more air, at a higher pressure.
real simple two type of tanks steel and fiber. steel tanks are cheaper bit are heaver. there are two type of air tanks for paintball c02 tanks that are steel and have a max working psi of 1800. there filled by weight (2-24 ounces). the co2 when filled is in a liquid form and turns into a gas. Hpa or n2 tanks is just compressed air. the three main psi for hpa are 3000/4500/5000. the last one 5000 psi was for some empire tanks that never took off. the size of hpa air tanks for paintball markers run from 13ci to 119ci in size. the small 13ci are for pumps and the 90-129 ci tanks are about the size of a 2 litter soda bottle. there are some odd sizes now 44ci and 51ci on the market and there the 70-80 ci tanks that are more round then they are long. trick with a tank is buying one for how you play and the weight and size you want. if you shoot a lot you may want a larger tank so you dont run out in the middle of a game. if your a front player you might want a small and light tank. A lot of players think they need big tanks and find out there too big and heavy. most players find the 68ci to be just about right.
Does an ursine quadruped void wastes in an arboreal environment?