Both amonia and nitrite are consumed by aerobic bacteria (i.e. in the presence of oxygen). Nitrate is used up in an anaerobic process (i.e. no oxygen). Bio balls are used in a trickle system to try and elevate the oxygen level as high as possible and have as much surface area as possible. Those bio wheel HOB filters you commonly see for fresh water tanks are trying to employ the same concept. This makes them very efficient at reducing amonia and nitrite, but the end product is as much nitrate as can be produced given the bio-load of your tank. Before we started keeping lots of corals, back in the late 80's early 90's, this was fine because our fish weren't as bothered by the nitrates. Regular water changes kept things in check. Now that we've started to focus on corals, which are more sensitive to nitrates, our focus has shifted, and trickle filters have gone out of vogue. There are many ways to reduce nitrates, most of which have already been mentioned...
First, water changes. This is the most basic solution, and it has other benefits. Be sure to try and match salinity and temperature. Frequent, large swings in water chemistry can be just as bad as to much nitrate or phospate.
Second, reduce your bio-load. Less feeding, less fish, less corals, but that's no fun. I found that by keeping a good pod population in my tanks, I can feed less than I used to.
Third, you can add plants (usually algae in a saltwater tank), because they use the nitrates. This is why we have refugiums.
Fourth, you can try to increase the surface area where anaerobic conditions exist. Think internal pore space in your live rock, deep sands beds, or a couple of different marketed media. The problem is we spend oodles of money increasing circulation which, by its very nature, keeps things as aerobic as possible.
Fifth, you could try a bio-pellet reactor. I use one, but have mixed feelings. The concept is that the bacteria growing on the pellets use the nitrate (and phospate) much in the way algae does. The tumbling action then physically abrade the outside of the pellet, and the bacteria are swept away to be collected in your skimmer. New bacteria grow, and more nitrates and phosphates are consumed. Rinse and repeat (continously). Great concept, execution is not always perfect, and not all bio-pellets are created equal.
Sixth, (and it's only sixth because I forgot to mention it, and I'm too lazy to go back and re-number) make sure your skimmer is working well. Skimmers physically remove the bio-load before it can be broken down by any of the bacteria in your tank. If you do this right, you can keep as many fish and corals as you want (within reason), feed as much as you want, and still have the benefits of a reduced bio-load. On my daughter's tank, the skimmer doesn't do much until right after I feed. Then it comes to life. To me that indicates I'm efficiently removing any excess waste from the system asap, and that means less nitrates in the long run.
So that's six suggestions. That's less than a buck a suggestion. I think I've earned the five dollars. Besides, I promise to use the money to buy Starbucks for you know who.
Cheers,
Chris
Performed a 20g WC three days prior yet it still rendered a number that's too elevated for my liking. 
