the fact that you have no ammonia, some nitrite, adn some nitrate, probably indicates that you are in the 2nd half of your nitrogen cycle. Read the sticky threads posted at the top of this section about cycling a new tank and setting up a new tank. You are on the road, but still have a little ways to go before your tank stabilizes and is really ready for fish.
If you have fish in yor tank already, then you need to change water. If I were you, and I had fish in the tank, I would do about a 1/3 water change every day for five days straight, as this should bring the nitrite level down to about 0.5ppm. Your bacteria colonies are growing ont he surfaces inside your tank, NOT in the water, so changing the water does not decrease your bacteria colonies, it only cleans up the water to keep your fish healthier through the cycling process.