Nitrite

This one definitely is something you do not like seeing in your water. Simply because it really can kill. I wrote earlier that it blocks Oxygen from being carried through the blood by some transformation in the hemoglobin structure. Other sources also claim that it may be directly toxic to the fish. Nothing much can be done except to do water change when it comes to nitrite levels in 'immature' or 'uncycled' tanks (it is a reminder of the importance of properly cycling your biofilter as it takes around 3 weeks to establish enough colonies of bacteria for nitrite to nitrate conversion, remember the bacterial reproduction takes 24 hours to form another cell plus the fact that ammonia to nitrite conversion is way faster than nitrite to nitrate conversion, so nitrite levels add up quite faster).

Just a quick preventive measure: Try a product called Nitra-Zorb by API (which is available locally) to reduce the 'risk.' Although Nitra-Zorb is rated for ammonia removal, the package insert do indicate that it does indeed remove nitrite and nitrate at a lesser degree. I have tried the product during start up (of some of my tanks in the past) several times and it did work.