Looks like were talking about the bulkhead fitting. It's supposed to seal where it passes through the filter body, it's strange that yours pinched out - unless someone was playing with it last year. :?
However, though it's supposed to seal inside the wall of the tank - it will also seal to the outside of the tank

You'll need to remove the multiport to make the fix, but you won't have to fully unscrew the bulkhead fittings. Once the multi is off (out of the way) unscrew the bulkhead fitting a couple of turns - what you're looking top do is create a gap between the nut and the filter body enough to put 2 new o-rings on and then you make your seal on the outside of the tank. You'll need to have 2 o-rings that will fit the pipe going through the tank body (any of the S200 bulkhead o-rings S200Z1-Z5 ought work - why they make 5 different ones, I just don't know but get 2 of the same). Once they're unthreaded you can gently stretch them over the nut and they'll tighten on to the pipe. Carefully retighten the nuts, you want decent pressure on them but don't want to have them 'pinch out' like your current one.
The reason for doing both is so that the multiport will attach evenly to the filter. If you can't get the bulged (bad) o-ring back inside the filter, cut it and remove it so it won't interfere with the new seal you're making.
The reason for not fully unscrewing the bulkhead fittings is that the receiving pipes may shift and not stay lined up and sand could get in the threads.
If you were thinking of replacing the sand, it would be preferable to have the o-rings where they belong and would be easy to do without sand in the filter, but I'm thinking you don't want to replace the sand, just to fix an o-ring
If I've been unclear in this or if you have any questions about what I've said here - all you need do is ask! :-D