Does anyone know a cheap way for students in middle school to test for dissolved oxygen levels in water?

I'm an intern trying to make lesson plans for middle schoolers, and one of them I was hoping to have them grow phytoplankton in a tank and then add certain levels of nutrients to simulate nutrient pollution in the ocean. Ideally, the phytoplankton would "bloom" (albeit on a very small scale) and then die off before being decomposed by bacteria and consuming all of the DO in the water.

My one issue is that I can't figure out a cheap way for students in public schools to test for DO. Does anyone have any tricks by chance?