I checked out a video from the library on setting up a freshwater tank. They suggest using more than one filter. I have a 20 gallon tank with a Penguin 125 and was thinking of adding another Penguin 125. Should I go with the same size filter or a smaller one? The current filter does not seem to create any current. The video also said that the second filter serves as backup and allows the fish to be fed more often. It said that by changing the filter media every 10 days instead of once a month the biology of the tank isn't disturbed as much. One filter media would be changed on the 1st of the month (the original) and the second filter media would be changed around the 15th of the month. Opinions?

It is good to have to filters, then if one burns out, or you have to completely clean one, or something, the other is "seeded," and your entire tank will not have to go though a whole cycle- saving undue stress on your fish. I have no idea why this allows your fish to be fed more often, unless the video means the ammonia output can be utilized more quickly by the nitrates. That willprobably be a waste of money to change the media out that often- are you using charcoal? If you are, then go with the recomendations on the cartrage. If it's jut "floss," then you shouldn't need to change it very often at all, just when it gets clogged. Then you can rinse it in tank water and put the same piece back in.
Another 125 should be fine.

Yes - the video indicated that two filters will clean the water more efficiently than one. Most references I have read say that you need to change the filter media monthy (when you clean the filter). The last time I changed the charcoal filter media it was brown and full of crud. I just rinsed it when I did the weekly water changes. The replacement filter media for the Penguins are only a few bucks. However, if I am not doing this correctly please let me know.

It is definitely better to have 2 of the same, you only need to stock one type of media.