If your tank is reading 0 for all readings i agree you somehow crashed the system. Possiably when you added 12 juvies all at once . I would recommend that you change 10 to 20 percent of your water daily or everyother day until your tank cycles. When ever you have unexplained deaths the most important thing you can do is to test the water and do a 50 percent water change as soon as you notice the death. This will often head off other deaths that will shortly occur. I would recommend that when the tank is cycled that you do a 20 percent water change (or more if needed) weekly when you vacume out the sand/gravel weekly. This will help to keep the water clean . Not enough water changes and vacumng is the root of most problems that involve bacteria and unexplained deaths.
Good luck.
CG