Testing for nitrogen levels

Question
My son is doing a high school science fair project on managing the growth of the brown tide algae by managing nitrogen levels in the water. Can you tell me the simplest way to check nitrogen/nitrate levels in salt water? Is there an inexpensive product we can buy (test strips of some sort?) Thanks!

Answer
It is impossible to test nitrogen in its pure form in aquaria, however you can buy a nitrate test kit at most any pet store for under $8.
Don't buy the test strips, buy the drop tests. Occasionally the test strips are a few bucks cheaper, but they have a margin or error equal to the entire range of the chart.

Nitrite is also in aquaria, that may be worth testing.