transfer of a danio (freshwater) from 1 tank to another

Question
Hi Mae: I have a 5 gal. freshwater tank [6 months now] w/ 2 purple danios, 1 yellow danio which is subject of my writing; 1 cory,2 glo-fish.  My 5gal tank looks beautiful and ALL fish are/were healthy! My son inherited a 20 gal. tank-5 months ago. On separate occasions  (freshwater) he put in a young lemon oscar, a small tiger oscar & young striped oscar. Within a month, each oscar died. Thereafter I bought him a tiger loach, & 2 dwarf gouramis.
Both of our tanks thereafter remained stablized and fine! Because there was so much room in his tank compared to my 5 gal. tank, I transferred 1 purple and yellow danio to his tank during the morning to watch their appearances and behaviors for rest of the day. I watched them for 2 days. Everything appeared FINE. The 3rd morning, very early in the morning,  my wife informed me that my yellow danio died.
Mae:what possible/probable causes/reasons could have caused my  yellow danio to die after being conditioned to the new tank in time-frame of 3 days? MY other (transferred) danio remained fine.  
You can understand how this surprised/startled & saddened  me-It was fine only 7/8 hours ago! I  immediately returned my purple danio back to my tank & promised myself never to transfer my fish again.
P.S. He's doing fine back in his tank. Water supply to both tanks come from the same water source. My son's Aqueon 20/30 gal water filter was in place. The only other difference was his  tank is slightly cloudy, Mine-crystal clear. Thanks Ken

Answer
Kenny,

I know close to nothing about freshwater and I dislike to turn you away but I don't even know what these fish are other then googling them. My only guess would be if the water is cloudy check your levels and clean your fliters. I would generally guess just cause one fish did fine doesn't mean the other will. I would recommend looking under the fresh water section and ask one of those experts as I have never had any interaction with fresh water and am not sure how to answer you. Sorry