Bio-Wheel filters are a very popular hang-on-back (HOB) filter. I have used them a lot and they used to be my favorites HOB. They aren’t anymore.
Pros
The Bio-Wheel is the main pro for these filters. The bacteria colonize the wheels because they are exposed to the oxygen in the air (up to 30,000x the amount of oxygen in the water). This allows them to work MUCH more efficiently.
Extra slide-in spots. This is great if you want to double up on their slide-in cartridges or use their media baskets (a step up above most HOBs to at least give you some extra media capacity).
Intake Strainer. These have great intake strainers. They definitely spread out the flow and even keep the grate even further away where the actual intake pipe connects. One of if not the best (Take notes AquaClear!).
Cons
The Bio-Wheels always seem to stop turning properly, negating the biggest benefit of these filters. Cleaning their holders can help, but it often isn’t enough.
Slide-in Cartridges. This is the norm for HOB filters and for good reason: it keeps you buying them forever. These are dinky, overpriced, and not very effective. As soon as they clog the water just goes over them. Compare this to the media in an AquaClear and you will cry (and you don’t even have to keep replacing the media in an AquaClear).
Noisy. Because of the Bio-Wheels these filters make a little more trickle noise than most HOBs (even with the tank’s water level completely full). It usually isn’t too bad, but if this is a factor for you at all this is not a good option.
Summary
Although not the worst HOB, they aren’t worth using. Definitely go with the AquaClears. If someone gave me a brand new Bio-Wheel for free I would sell it on CraigsList.