I've got currently 7 tanks in operation ... If one fails, it'll simply get a bit tighter in the other six. Besides, two tanks have double filtration (one extra is an Eheim 2262). All in all, I hope I will never need it.
I have a back-up tank that runs continuously with only 1 cory catfish & 1 bristlenose pleco to keep the nitrogen cycle maintained. But I mainly run the tank as a nursery/quarantine tank.