No, they can't. A nitrate molecule is probably larger than a water molecule, but it's not the sort of thing you filter. IANAChemist, but I'd guess what you want to do (it's NO3 [nitrogen/oxygen) is break it down chemically, break the bond, which releases nitrogen as a gas, I'd guess, which is what the bacteria in your rock and sand do. This is one reason why live rock is a better cleaner than, say, filters, which only trap gunk, but that won't lower your nitrate the way good old biochemistry can do. Most of what you want to do in your tank involves 'breaking down' the stuff, so it changes into two good things (nitrogen gas, which being lighter than water---leaves; and oxygen, which the fish can use. If your curiosity still wants to know more---ask over in Reef Chemistry and some real live chemists can give you an answer involving angstroms and molecular weights and actual description of the molecule compared to, say, water.