well, first off, looks like the return BH is upside down. but you're saying that's not leaking. I'd still reverse that if you can. the bulkheads should be set in such a way, that the threaded part points DOWNWARD, and the nut is threaded upward into the tank.
order should be like this
[top of tank]
[threaded part of bulkhead]
[gasket]
[hole in tank]
[BH retainer nut]