Take a look at the bulkhead, where is the gasket? It should be against the flange and not the nut. If the gasket is against the flange it's installed incorrectly and needs to be fixed.
If the gasket is in the right place, try tightening the nut a bit by hand. Do not use a wrench of any sort, but if you can move the nut with your hand then you can snug it up. If that doesn't work, look for cracks in the plastic...