So you have two fish in a 20 gallon tank?

What filtration have you got?

When do you do water changes and how much water do you change?

In a healthy tank/fish a few small power outtages shouldnt kill a fish.. either you have problems with your set up or maintenance routine or the fish wasnt healthy in the first place.
You should quarantine new fish to stop diseases spreading to your existing fish. This means isolating them in a seperate tank for at least 21 days to make sure they are not sick, and to treat for anything if they are sick.