You still weren't specific in answering my question. If you've been using the same cartomizer for "a few weeks", that carto has more than done its job.
There are carto punch tools available at most e-cig vendor websites, but I don't recommend them. The majority of these tools invariably distort the shape of the carto from round to oval. When the deformed carto passes through the endcaps, it causes unnecesary trauma to the rubber o-rings in the end caps, as you found out with your experiment.

carto deformed from tool punching
Slotting with a dremel tool does the same thing. This causes sharp edges in the carto which also can damage the o-rings.
O-rings are very important to a tank. They contain the negative pressure inside the tank to allow juice to enter the carto from the tank each time you draw. They allow the tank to fit snuggly on the carto so that it doesn't slide off and spill your juice. They also keep juice in the tank without leaking.
If at all possible, use pre-punched cartomizers as they cause no damage to the o-rings. This link has over a dozen vendors that sell these, some offer 3 holes punched.
Pre-Punched Cartomizer Vendors
Get a tank tool like the slap-yo-moma tool, which further protects the o-rings from the tops of the cartomizers which are also sharp.
Unfortunately, it sounds like you have ruined you current tank's o-rings, which will need to be replaced. I don't know if you can get specific replacements, but auto part stores likely have them in that size.