ok.. here's my 2 cents.
prepare ahead of time for emergencies..
foam filters with air stones and battery air pumps or a stand alone generator for power outages..
Keep basic first aid meds on hand all the time.. aquarium salt, melafix, some kind of anti-biotic.
Spare heaters, filter parts for emergency repairs, nets for every size fish, PVC pipe in various sizes for moving catfish and fish with bony fins that would tear a plastic bag or get caught in a net.
never rely on a single filter in any tank, always have backup.