How high are the nitrites?
TTA discussed doing a "diluted" test to see the result. Doing this is fairly simple. Tap water should contain 0 nitrite (or very close) and if you put 4 ml of tap water into the tube and 1 ml tank water, and test that - then, whatever result you get can be multiplied by 5, to determine your level.
So, if your diluted test comes back at 1ppm, you are dealing with 5ppm nitrite. A test of 5ppm for the diluted water means a level of 25 ppm.
Personally, I'd do a water change if the nitrites are any higher than 10ppm. 50% would halve the concentration, 75% would cut it to 1/4 of the original value.
I'd dose the ammonia at no more than 1ppm daily until the nitrite hits zero. The nitrite bacs will catch up, but they can't handle huge quantities of nitrite - kind of like the difference between drinking from a garden hose and a fire hose.