Huge wet/dry system will do exactly what it was designed to do... get rid of ammonia and nitrites. These forms of nitrogen are converted by aerobic bacteria. Wet/dry is perfect for this, but to get rid of nitrates, you need an anaerobic area in your tank. Nitrates are converted to free nitrogen by anaerobic bacteria. Protein skimmers will remove waste from the tank before it has a chance to be broken down into forms of nitrogen. Also...nitrates will be removed in direct proportion to the amount of water changed. If you have 40 ppm then changing half you water will result in 20 ppm. If you do 5 gallons on an 80 gallon tank, then you nitrates will only reduce by 1/16th. Meaning you will go from 40 to 37.5 by changing 5 gallons of water. Doing water changes more often or in greater amounts will help get nitrates under control. To keep them under control, look for the cause..too many fish, overfeeding, no protein skimmer or not big enough protein skimmer, something dead and decaying in the tank, dirty filter media or sponges.
Hope this helped a little.