As I mention many times in this thread, I do not and will not use slip bulkheads. I have seen far too many of them cut apart and thrown away. As redfish mentioned though, if the slip is on the inside you should not glue the standpipe or fitting in as it will need occasional disassembly and cleaning. If it is external, and slip is what you have and your only choice, not mine though, then do glue a union somewhere close. If you ever have to remove it it will still require cutting apart but you can disassemble it. I would find thread by thread bulkheads if it were my tank so you can unscrew any fittings in the future and not have to waste a bulkhead. Silicone does not permanently adhere to plastic or glass and the seal can be easily broken. I would remove it and take the time to investigate why it was dripping and do a proper fix. It's usually some excess plastic in the threads or the flat flange surface that needs filing or sanding smooth.