Thanks very much for your advice Mustafa.   

  I'll try the neutralizer.  In fact I used to let tap water expose under sunlight by the windowsill for a day before I pour it into my tank.
I wish to find out the cause of their disappearance so as to avoid it next time.  Here I have three problems:
(1) They worked well in the first ten days.  I don't understand why they didn't die earlier (I'm not cursing them);
(2) So far I've never seen any dead bodies, that's strange;
(3) I wonder if my one-and-a-half-month-old cherry babies ate them up.  However, I wonder why they didn't eat them earlier.
Oh, the last two baby tigers have gone.  As expected, I can't see any tigers today.  
 
   
I was full of hope when I saw over 50 babies and worried about how to deal with 50 adult tigers later.  It turns out that it's a false hope.