nitrate level

Question
I have a 125 gal saltwater aqurium, it is lightly stocked. I did a 30% water change on the tank and 24 hours later checked the ammonia, ph, nitrite, nitrate & salinity levels.  The nitrate level was more than double what it had been before the water change (it was 40 ppm before, 120 ppm after).  What could cause the nitrate level to increase so dramitically after a water change, usually a water change lowers the leve.

Answer
The nitrate level was a little high to start with but to jump like that the water you added must have contained very high levels. Is it possible you could have misread the results? Definitely does not seem logical unless, as I said, the add water was from a source extremely high in nitrates. Gerry.