Hello Newbie here, I am having trouble establishing a Bromine reserve in our spa. I first got the PH and Alk in line.... then added a pre-measured pack of Bromine to build a reserve. Float is filled with tabs. When I check the water with test strips or a DPD test the water reads zero. I added another pack of reserve builder today (last pack was added 2 weeks ago upon start up) the spa is 415 gallons, bromine packs are for 400 gallon spas.

Anyone have any ideas ?

Have you used your spa yet? When you first filled your spa with fresh water and adjusted your PH and alkilinity, were you instructed to add anything else but the bromine? When I instruct my customers when they are just getting started and they are going to use bromine as their sanatizer, I tell them that they should add Sodium Bromide first, before adding the bromine. This prepares the water for the actual bromine. Did you get a starter chemical kit when you purchased your spa? You can get these kits from any Spa dealer or Spa chemical outlet near you. Any starter kit that I have ever seen that uses bromine as the sanitizer instructs the user to add the bromide first.
Hope this helps! Good luck!

Try just shocking the water.