Inside the tank weir plumbing should be removable. Slip fittings just set the pipe in the bulkhead and threaded fittings screw it on but don't use sealant. For outside, you want it permanent. So glue slip fittings and use sealant with threaded fittings. Slip fittings almost never leak, threaded frequently leak. I make a stub fitting coming out of the bottom of the tank and use a PVC compression fitting from there. This allows me to easily remove the bulkhead and also turn the plumbing as needed within the compression fitting. Here is a video I made that may help.