Do biowheels make protein skimmers ineffective? Do I need a protein skimmer with two biowheels? Which is more important to have; protein skimmers or biowheels?
First off, they are 2 different things and not really related. A filter with the biowheels is there to filter the water both mechanically and biologically. A skimmer removes dissolved organic compounds (DOC
So, to answer your questions....
2- Yes, a skimmer should be used on a 40g tank
3- Both, but if you have live rock you can eliminate the filter with the biowheels as the rock is the biological filter.