If that was your TRUE Nitrite reading I think your tank mates would be showing some signs of stress and even death. Can you rinse the tube really well and redo the test.
I believe something may have contaminated the test tube. That CAN'T be the true readings if all your fish are fine and eating etc... I'd try it again.
"When creating an aqua scape, strive for the beauty of imperfection, as its art and there is no wrong or right, it's left open for interpretation" James Findley