BCDMH contains both chlorine and bromine. Both are released into the water where the chlorine oxidizes any excess bromide into bromine. Tablets will build up bromide levels very slowly. If you are using MPS to oxidize the excess bromide into bromine for shocking and there isn’t enough bromide around, you’ll get a mixture of bromine and persulfate in the water. Establishing the bromide bank up front just ensures that your water always has enough bromide in it so that shocking with an oxidizers will create the needed bromine levels. I'll have to check but I think most are Bromochloro-5, 5-dimethylhydantoin . And I guess they all have a little chlorine in them. Thank you!You can add as much sodium bromide as you like, it’s inert until an oxidizer converts it to bromine. So even if you have 100ppm sodium bromide in the water, if you only add enough bleach or MPS to generate 4-6ppm bromine, then that’s all that it will create. The oxidation/reduction reaction of chlorine and bromine happens quickly and completely - 1 mol of chlorine oxidizes 1 mol of bromide into bromine. The chlorine gets reduced from chlorine to chloride. Chloride is inert and no longer reacts with anything.