Never never never walk away from an aquarium when you’re filling it with water from an RO unit or a container of saltwater. While you may have failsafes in place, and your memory may be as sharp as a tack, chances are you will forget it one time or another and fill your aquarium with a whole bunch of water. For a freshwater flood, this obviously runs the risk of lowering the aquarium’s salinity and causing significant harm to your livestock. A saltwater flood, fortunately, is much less stressful on the livestock, as the water chemistry doesn’t change much. Only the volume changes, and that can be easily remedied by siphoning some back out. In either situation, besides damage to the aquarium, a flood can damage your home (or wherever you keep your aquarium). So, if you’re filling, stay with it and devote plenty of attention to the task at hand to prevent huge issues.