Can I measure Total Residual Oxidants in our reef tank (expected results of 0.05 ppm)?

On a small scale, the HI761 Ultra Low Range Total Chlorine Checker would work well.

The HI761 Ultra Low Range Total Chlorine Checker® has a range of 0 - 500 ppb, so the expected concentration of 0.05 ppm (50 ppb) would be well within the range of the checker. The checker has an accuracy statement of ± 5 ppb or ± 5% of the reading, which is appropriate for that measurement range. 

This checker will detect and measure strong oxidants such as bromide and ozone in addition to the total chlorine. Thus, the result would represent Total Residual Oxidants, if there are strong oxidants present in addition to chlorine.