You can reduce the creation of nitrate by feeding less and removing as much debris mechanically as you can. Rinse cartridges, gravel wash substrate.
Nitrate is the end product of the FW "cycle". All food and dead plants, fish and algae in a FW tank ends up as nitrate. And, unless you have plants or algae consuming nitrate, the concentration will just keep going up. Changing out media may slow the conversion of ammonia and nitrite to nitrate, but as nitrate is the least toxic, that is not a good thing. Rinse media, but replace only when it starts to fall apart or you need fresh carbon to remove meds, smells, or oily film.
Changing water is usual method to remove nitrates. And its a concentration math. So to reduce nitrate from 80 ppm to 40 ppm, you have to change half of the water. Small changes are multiplicative, rather than additive. So 20% + 30% doesn't equal 50%. Its (1-(.8*.7))*100% = 44%.
There are a bunch of new products marketed to aquarists to get "rid of nitrate". They have names like "sludge away" and claims like "never change water again". Do NOT use these in a tank with fish alive in it. They run the cycle backwards from nitrate to nitrite to ammonia to nitrogen gas. The higher your nitrate level, the more dangerous it is to convert it back to ammonia. They also attack slow-decaying crud in the tank like dead algae. The nitrogen gas is safe and soon the water will be nitrate free, but by then all the fish are dead. In theory, you could use such a product constantly and have only low levels of ammonia in the tank, but I have heard too many horror stories to consider risking it. IMO, they are only useful if you are cleaning an old filthy, fish-free tank.
There are few other methods that have some utility. One is to use a substrate that will chemically remove some nitrate. These have some use in high pH and hardness cichlid tanks.
Another is an ion exchange resin. An ion exchange resin in a pouch in the filter will take up nitrate and release another ion, usually chloride, in its place. There are more that a few drawbacks here, too. You add salt to the water, not good for many plant and inverts. When the resin is exhausted it stops taking up nitrate. But more of an issue is that these things aren't that selective. They often take up nitrite also. If you consistently remove nitrite as it is created, the nitrite to nitrate bacteria will starve. When the resin is used up (you can recharge with salt), you suddenly have a huge nitrite spike, which again can kill fish.
If you go with ion exchange or any other chemical nitrate removal method, you need to be testing all the time until you work out a process and schedule that keeps your fish alive and nitrates controlled.
Testing nitrate is a decent way to figure out what your water change schedule should be. Keep nitrate low, and likely all the other bad stuff is in check also. You will have to ramp up water changes as fish grow and get fed more.
Last edited by emc7; 05-08-2012 at 05:53 PM.