Adding New Fish: Best Practices?

[RobRocksFishTank]

Perhaps it is not necessary to have two filters running. I have a spare filter ready to plumb in but I would use the filter material from the failing filter to put in the standby filter if the first failed.

 
Necessary?
No
 
Advisable?
Yes
 
Having more than one filter has many advantages but for the purposes of this discussion lets stay with the redundancy in case of failure.
 
Failures don't usually happen at convenient times. In fact, unless you spend most of your day near your tank, it will fail when you are not there to address it right away. For me, my tank is unattended for most of the day, every day. There are times when it is unattended for more than a day. For me, having secondary filters running could prevent a disaster, if the primary one fails.

 

[Byron]

While I understand the reasoning behind two filters, I personally have never done this.  You are probably more likely to experience a power outage than filter failure, and then you have no filters at all.  Only once in 25 years have I had a filter fail, and that was when the power went off for a few minutes when I was not home, and I returned to the smell of burnt metal--the HOB filter had run dry when it restarted.  Never used one of those again.  But it was the fire hazard far more than the water quality in the tank that concerned me.
 
If an aquarium cannot manage without the filter for a few days, it is probably overstocked or inappropriately stocked.  After all, tanks with no filter at all work fine.  I've had one for over a year.  And with two filters there is the issue of too much water flow or too much filtration, which can be just as bad.
 
Byron.