You should use Weld-On #4 to seep into the seems where you connect the acrylic. It is a solvent and will actually "melt" the acryllic together. After that dries, follow up with the Weld-On #16 as a fillet at every joint. It has more solids in it and works more like a cement. A good method is to make the acryllic pieces slightly longer than needed, then router off the excess after you weld it all together (as noted, above).
If you need any pointers, just send me a message. I came darn near to building my own tank out of acryllic before I got scared off. I have built a few acryllic items and repaired a broken skimmer, too. I have a can of WeldOn #4 you are welcome to use if you like. But it is over a year old and may not be the best now, though. You can buy a new can of #4 for about $8 and an needle applicator bottle for about $4 at a local vendor. I can look up the info on the place I used if you need it.
-Scott