Speaking of filter cartridges- When they get clogged up if you will rinse them in used tank water (instead of replacing them) and put them back in the filter you will not only preserve your beneficial bacteria but they can be used for awhile longer and save you money. When they get really ragged and you have to replace them replace only one at a time. You can also put a piece of the old filter in with the new one for about a week to seed it.
Well I have great news. I got the bigger Penguin for less than the smaller one. Petco honored an online special and did an exdchange for me!! Now, I just have to make sure that it isn't too big (physically) for the tank. I don't have the tank yet-- ordered on ebay and having trouble with "the guy" in getting it here! (Long story, see another thread.)