An all-in-one tank should have built in filtration capability. Marineland makes relatively inexpensive systems and you often get what you pay for in this hobby.
The Biowheel is going to be a headache unless you clean it regularly. Biowheels are intended to provide biological filtration (converting relatively toxic ammonia excreted by fish gills into much less toxic nitrate). Live rock provides the same capability so your "filter" for a reef tank only needs to remove debris. Can you remove the Biowheel and replace it with aquarium safe sponges? Many small all-in-one tanks use sponges as debris filters in their first compartment. I ran a 24g nanocube as a mixed reef tank for quite a while this way.
As others have suggested, a skimmer will be a good addition to your tank if there is space for it. You can run a reef tank without a skimmer, but I would try not to.