just get an underwater pond silicone, its from germany the one I use you can go around the inside of the bulkhead with it why the tank STILL IS FULL. so no need to drain and its 100% fish safe thats why it costs a bit

I have used it to fixed crack tanks etc with fish in them and no issues. its very painful using underwater if your not very handy. My friend who had to use it found it easier to apply a thick smudge to his finger then run this around vs putting the whole gun under water.
only other option I can think of but I doubt you would want to do it, remove the bottom of the bh, water shouldn't drain any faster if the top part is installed right maybe get someone to apply light pressure to the side in the tank just in case. then get a new gasket apply the black silicone to this gasket and push it hard onto the base of the glass where the BH is. then normally people run a light bead around the inner hole where the threaded part is after that you put the base back on the last few turns will be hard as there will be some silicone there but i have never seen a bulkhead leak when done like this. Only issue is removing them you need to use a bit of effort.
I would check as to why it is leaking, I dont use any silicone or glues on my bulkheads and never had one leak, so something must be going wrong with yours.
http://www.absolute-koi.com/subcat546.html
^^^ This is the one I use at work/home. works wonders just be careful with this one once it does dry its like cement, its much stronger then most silicones, I have issues cutting it with a new knife so dont go too crazy

can also use it to on metals to glass, at work we started silicone aluminum edges to tanks instead of the black plastic ones we believe it gives the tank a bit more strength.