When encountering a leaky bulkhead, one might assume that it is fine to dab a little silicone caulking around the fittings or even around the flange to stop the water from dripping. On the other hand, one might think it is ok to grab a big wrench and tighten down on the fitting. While either may fix the problem at hand, neither one of them should be used or even considered. The caulking is a temporary fix at best, and the leak will find its way through. Not to mention, some caulking may not be aquarium safe, so there’s that issue as well. As for tightening the fitting, it should only be done by hand. If you use a wrench, you run the risk of messing up the runner gasket, only making the leak worse.