Plumbing up a new aquarium and want to make sure that if the time ever comes for you to remove equipment or tear the system down for a relocation (be it temporary or permanent) you won’t have to cut off all of those expensive pipes and fittings? If so, the only way to fly is to use lots of unions and threaded fittings wherever you can. The union fittings, and the all important double union block valves, will be crucial when it comes to making your plumbing modular, as they can be unscrewed for easy removal of whatever is connected to them. As for the threaded fittings, they can be reused multiple times, where slip fittings are glued into place and can only be used one time, as the glue is impossible to break (or so it seems). That all said, slip fittings are still the preferred plumbing for aquarists, as they don’t leak if done properly and offer a better long-term starting point for your system.