The Penguin 150 biowheel is a very efficient biological filter. I am a huge fan of the Penguin biowheels, but there are drawbacks that you need to be aware of if using these on a marine aquarium.
First, recognize that one of the primary goals in a marine aquarium is to keep Nitrates at or near zero. The goal of the Penguin Biowheel is to break down waste, with an end result of Nitrate. These 2 concepts are in conflict with each other.
Another issue with the biowheel is related to alkalinity. One of your most important measures of stability in a marine aquarium is alkalinity. The biowheel is a biological filter, which means that alkalinity is being depleted during the nitrification process. This is another hurdle that must be overcome.
If your aquarium was larger than 20 gallons, I would be completely against this idea. However, in tanks of 20 gallons and smaller, the use of a biological filter is doable, provided you make a few simple adjustments to your normal routine.
First, you want to use activated carbon much more aggressively than in freshwater. You should literally change the filter pads out for a new pad every week, 2 weeks at most, regardless of how dirty they appear to be. I would highly suggest that you cut the top of the filter sleeve and add additional activated carbon. The reason for this is simple. Activated carbon absorbs organic acids. Organic acids break down into nitrate, and deplete alkalinity. If you can utilize the carbon to remove organics directly from the water, PRIOR TO THE WATER COMING IN CONTACT WITH THE BIOWHEELS, then the amount of Nitrates produced will be considerably less.
Another adjustment will be the cleaning of filter pads. You will want to rinse the pads DAILY. This will remove small particulates of waste that become attached to the pad and begin to decay, resulting in increased nitrates and phosphates. Even if you can't see dirt accumulation, there are small organic particles that will be removed with a daily rinsing.
The next consideration that will be very important is the depth of your substrate. You want to use a reef grade aragonite sand, at a depth of 4'' to 6''. This depth will provide for ideal denitrification, which is the process of Nitrates being biologically processed into nitrogen gas. Nitrogen gas leaves the system naturally and is harmless. This denitrification process does not occur in the biowheel. The deep sand bed is required. You will find this very helpful in controlling Nitrates.
Finally, be sure to test Nitrates weekly, as well as alkalinity and calcium. This is highly important when using a filter that is designed to intentionally increase Nitrates and lower alkalinity. You will need to make adjustments as needed.
I spend some time explaining these concepts in greater detail on this thread: http://www.tropicalfishkeeping.com/m...shwater-31955/