What kind of filter for what size tank? How many fish are in the tank? Most of the time if the filter is the right size for the tank than there is plenty of oxygen for the fish. However, if you are carrying a heavy bio-load than the filer will not be able to handle the amount of oxygen needed for all of your fish. In that case you can do one of two things or both. You can add another smaller filter and/or add a bubble wand to aggitate the surface of the water to break oxygen at the surface for the fish. Believe me, you can never have too much oxygen for them. I'm a big believer in giving more filteration than what is rated for the size tank and I also believe that understocking a tank is way better for the fish in the long run, not to mention that it is also better for the pocketbook. In case of a power failure there will be no filteration for the fish because the filters will be off. That means less oxygen for the fish. If the tank is understocked than there will not be as much oxygen being consumed by the fish and they have a better chance of not suffocating during a power outage. Well, I have gone on long enough. I hope this helps. I'm sure there are others that will chime in too.
Be happy!
Marinemom