Many people debate the effectiveness of bio-wheels, but owning a few myself I have seen that they, indeed, do their job. Once you understand what they do, it only makes sense that it makes an ideal site for bacteria to use as a home. It follows the basic principles of wet/dry bio-filtration, in which the media is alternately exposed to water and air. This gives them optimum oxygen levels and they are able to better do their job of converting nitrogen.
I own three different types of bio-wheel filters and have an opinion on each. The Penguin 125 are cheap knock-offs better left collecting dust. I was very unhappy with it's design, although it did offer geniune wet/dry filtration. Noise and extreme surface agitation (bad for planted tanks) were unable to be compensated for. I tried to "jerry-rig" several features and none worked well. I saw much room for improvement with this design, to say the least. Emperor 400 is a quality investment. This is the cadillac of Hang-on-back filters. It has all the features and the only real drawbacks were inherent to almost all HOB filters. This is still not a good filter for planted tanks, since it tends to "gas-off" alot of CO2, further increased by the bio-wheel technology. Excellent for fish-only tanks, IMO! I also have a Magnum HOT Pro (with bio-wheel) on the tank with the Emperor. This filter is awesome when you know how to use it! It is quiet, does not drive off alot of CO2 (as long as you don't use the bio-wheel attachment), and has many attachments which are used to "convert" it for different uses. I use mine as additional mechanical filter and use the micron filter for periodic water polisher, as needed. The media tray is the source of most of the negative talk regarding this filter. You must understand, it's not build to be the best bio-filter, which makes the bio-wheel attachment such a great feature for it. I have been hard pressed to find a more versatile filter, anywhere! Canisters are my new favorite, but aren't nearly as versatile. You can move this filter from tank to tank, polishing each tanks' water with minimal hassle.
Other filters do a great job, also. It really depends on your needs and what type of fish, setup, and other equipment you are going to be using. I wouldn't suggest any bio-wheel filter for a planted tank (more than a few plants). The tendency to lose CO2 comes from it's excellent ability to oxygenate the water! I no longer use airstones in my tank with bio-wheels. One of the great things about the bio-wheel is that when you break down and clean your filter (in-depth) you can leave the bio-wheel floating in your tank, and the bacterial colony isn't affected by your cleaning. I alternate cleaning my filters every month, as prevention and upkeep will protect your investments.
Hope this helps a bit!
