Holy shit! 400 gallons eh? Yeah, PVC glue wont work on HDPE...appearently, neither will any other kind of glue known to mankind at the moment...While the caps themselves are PVC, the IBC itself is HDPE plastic. I didn't use anything and none of mine leak. I did however use a bunch of 3M E5200 Polyurethane Sealant/Adhesive on various parts and pieces. PVC to LDPE, PVC to HPDE, HDPE to LDPE blablabla...and it seems to be working thus far...
If your talking about the lid of the IBC, two things. First CLEAN the o'ring of the cap! Then I used GE waterproof silicone and ran a bead around the inside of the cap over the o'ring and mine have yet to leak.
I didn't use anything on mine either - then after I noticed the cap was leaking, I had to take out the gravel to tighten the cap. Before I tightened the cap I used a lot of plumbers tape (the very thin white non-sticky type) wrapped around and around - once I retightened the cap it only drips now and then. Since mine is outside in a greenhouse a few drips don't concern me too much. It also seemed like after a few days my system leaked less than the first couple of days (i.e. less drips here and there at connections). Perhaps algae gets established in the connects enough to assist in sealing the connections?