It depends on the Bleach that you use. All different bleach companies use a different percentage of bleach. With Clorox regular bleach (not splash guard) I use 1 drop of bleach for every 2 gallons. I add that back in every 5 days. You don't want to exceed more than 2.5 PPM of chlorine or you run the risk of killing your plants. I like to add about 1 PPM of Chlorine and let it run down. If I start to see any problems with any plants, I get out my Chlorine meter and test the PPM of Cl in the system. If you have a pathogen, you will see the Chlorine leave the solution faster than normal. The way that I know this is because I've done lots of testing with it over the years. A chlorine meter is very cheap.