There are several links to help with all kinds of projects in the DIY sticky's. I'm almost possitive that there are links on how to fix leaks for both glass and acrylic tanks.
Your right about needing to use weld on. I use a #4 or #16 depending on how big the leak is. #4 is a solvent that melts together the seam. This is what is comonly used to assemble acrylic aquariums initially.
#16 is a thicker version that is used for filling gaps and irregularities.
I would recomend using #4 if you can. (or try it first) you can always use #16 if #4 doesent work for you.
Either one will haze out the acrylic so be careful and tape off the area around the leak.
good luck
I didn't know how good #4 was, so I used #16 to repair mine. I had a stress crack in a corner, so I glued small pieces over it. Not the prettiest, but it works, and I am looking for a piece of driftwood to cover it.