Howdy nznene, When I get the root rot (or at least when my plants do! LOL) I add 1 to 2 cups (0.24 l to 0.48 l) of 3% hydrogen peroxide in my six gallon (22.74 l) reservoir. But the first thing to do is to determine why the root rot is forming. All light should be blocked from the reservoir, the water kept well aerated, and the water temperature should be kept between 62° F and 70° F (17° C- 21° C). I tried the Hydro Guard several grows back and I did not care for the results. I have not needed it since I started keeping the light and heat out, and the air in (lol).
Good luck with Your grow.
P.S. I almost forgot, start by draining the reservoir, them re fill with pH'ed water and the H2O2, let it stand for an hour or so, the drain and re fill with the nutrient mix.