When setting up the plumbing on your aquarium, you have to be especially careful when assembling the bulkhead fittings and installing them. For starters, the surface where the bulkhead is being placed needs to be clean and free of debris. Additionally, the rubber gasket needs to be installed on the flange side of the fitting. Once the fitting is in place, tighten down on the nut by hand, making sure to not overtighten as water will likely leak by. One thing to also keep in mind is that you should never tighten down on the plumbing with a wrench, and should never have to use silicone on the fittings.