If a bulkhead fitting is leaking there are only a few possibilities, so check each one:
1) Are there any cracks in the plastic body of the bulkhead?
2) Is the rubber gasket flat and smooth on both sides? If it has any scratches, rough spots, gouges, or thin spots, then it won't seal.
3) Is the glass perfectly flat and smooth where the gasket touches it - no deep scratches?
4) Is the surface of the flange perfectly flat and smooth, without scratches or plastic seams?
5) Is there any sand, dried salt, hair, or anything else, getting caught on either side of the gasket, preventing the gasket from laying down completely against the two hard flat surfaces on each side of it?
6) Is there a soft seal between each pair of hard surfaces? Where ever you have two hard surfaces that come together, you must have an elastic seal (gasket, silicone sealant, etc.) between the two hard surfaces or water will sneak through. That is true unless the two hard surfaces are "welded" together (like with PVC fittings bonded with PVC cement (solvent)). Look at your bulkhead, there are two places where hard surfaces come together that must be sealed. The first is sealed by the rubber gasket where the glass and the bulkhead flange come together. The second is where a pvc fitting connects to the part of the bulkhead that sticks outside the tank. Assuming that the gasket is working, your leak must be coming from this connection. Is that connection a slip fitting, or a threaded fitting? If slip, you need to use PVC cement (or at least silicone sealant) between the two parts that slip together. If it is a threaded connection, you need to use two wraps of teflon tape on the threads, and then screw them down tight. Or, instead of teflon tape, you could use silicone sealant.
7) Is the nut tight enough to press the gasket between the glass and the flange? If the gasket is very stiff, you might need to tighten it down very tightly to get it to seal.
That covers all the possibilities. One or more of the above is your problem.
Here's a Picture of how the parts should go together (the wet side would be the lower side/bottom of the picture) If the gasket isn't leaking, it must be leaking where the black and white pieces come together. PVC cement, teflon tape, or silicone sealant can seal that joint.