Nitrates to Nitrogen Gas

[Kadina]

I saw someone mention here that bioballs in a wet/dry filter can lead to high nitrates. Being curious, I did some goggling to learn why and found a thread that said:

Water from your tank is sprayed or trickles over them creating a high oxygen enviroment This creates almost ideal living conditions for certain bacteria that break down amonia/amonium to nitrite. These same conditions are good for another set of bacteria that utilize nitrite and leave nitrate as waste. These 'trickle filters 'are very effecient for this part of the 'nitrogen cycle' . These filters are not however good for the final step in the 'cycle' which is to convert nitrate to nitrogen gas. This requires another set of bacteria that do not thrive in the oxygen rich enviroment of your trickle filter. Thus the water coming from these filters are high in nitrate.
I have never heard about this third type of bacteria before. I was under the impression that the only way to remove nitrates is to do water changes. Is that quote just a load of baloney or is there any truth in it?

[Alfcea]

Yes. These types of bacteria do exist and are called "denitrifying" bacteria. In nature, they occur under anaerobic conditions but they are very unlikely to grow in fish tanks because these environments are usually aerobic (with lots of air/oxygen). So, you are right. For our purposes, the only way to remove nitrates is by doing water changes (or planting heavily)... but nature does have other ways to achieve that end too!