Ah what the hey. Here we go...
In order to better understand nitrates, you need to understand the nitrogen cycle.
When you first establish your tank, you won't have any bacteria in there. Nothing. Notta. Zip. You need to kickstart the system by adding some of the bad stuff, also known as ammonia. It's the first step in the process. Ammonia is released into the tank through decaying matter, such as uneaten fish food, rotting dead animals and fish waste (pee and poo). To simplify the explanation, ammonia eventually turns to a bacteria called nitrite. The nitrites feed off the ammonia. The more ammonia you have, the more nitrites you'll have.
In a few days, the nitrite is converted to Nitrates. The nitrates feed on the nitrites in the tank. See? Ammonia -> Nitrites -> Nitrates. Nitrates is the last step and this is where the problem lies, because there is NOTHING in the tank that consumes the nitrates. The only way to get ride of it (other than adding vodka or vinegar to your tank which seems risky to me) is to export it or prevent it. You export it/get rid of it manually, is to do water changes. Stay with me here. If your nitrates are 160 and you do a 50% water change, that will drop to 80ppm. Another 50% water change it will be 40ppm
It goes down, yes, but NOT if there's something in your tank that is still releasing large amounts of ammonia. Because remember? Ammonia turns Nitrite and nitrite to nitrate. So if you do a 50% water change and say you remove 80ppm of nitrates. If something in your tank releases 80ppm of ammonia, that 80ppm of ammonia will turn back to 80ppm of nitrates. So you're back to where you started. You need to figure out where the ammonia is coming from. Here are a few examples of where large amounts of ammonia can come into play.
1. A new tank with dry rock that has not been properly cleaned, which is releasing waste, die off and junk.
-This will take a long time to clear up and can only be fixed by either removing all your rock and starting over, or waiting it out patiently for 6+ months
2. Those blue filter pads can harvest nitrates.
-They need to be replaced every 1-2 weeks. If every 2 weeks, you at least need to rinse then throughly every 3-4 days.
3. Biowheels can hold nitrates
-Take it out. Those are for freshwater tanks. Not Saltwater
4. Not enough live rock or uncured live rock
- Live rock helps to remove nitrate by releasing it into the air in the form of a nitrogen gas. Live rock HELPS, but you can't rely on it entirely. You need at least 1-2 pounds of live rock per gallon of water in your tank. Also, it takes about 5 months or so for your rock to "cure". Only when it has had time to "cure", will it be of any use.
5. No Skimmer
-Skimmers are nearly essential in a saltwater tank, as they remove waste BEFORE it has a chance to turn into nitrates. If you don't have a skimmer, BUY ONE.
6. Too many fish/Over stocked tank
-Fish poo and pee. If you have too many fish and your tank is overstocked, they will be releasing ammonia->nitrates quicker than you can remove it. In this case, get rid of some fish or get a bigger tank.
6. New tank
-New tanks often have problems with parameters until things level out. This maturing process can take 6 months or more, depending on your setup.
If I've missed anything, someone please correct. Hope this helps.
So the first question is "How old is your tank?"