RIP Taco

If you didn't hear we recently got a pet fish. My daughter was super excited and this was a test to see if she could care for a pet before we get a bigger pet like a cat or dog. She loved Taco and was so excited to see him and feed him each day. About 4 days into Taco's arrival he didn't start looking so good. We used R.O. water, used the fish water conditioner, a 5 gallon tank with pump and Beta food.  He would start hiding in the fake plants all day. Then he worked his way behind the filter pump and hide behind there. Then I knew something was wrong when he wouldn't eat his food anymore. That night I Googled and even though his water looked great but I planned to change it out the next morning to see if that would help. By the next morning Taco had gone to the big fish tank in the sky. Lovelyn was super upset. We still can't figure out what we did wrong. Was he sick when we bought him? From what I hear Beta fish can live in a freaking mud puddle....how the hell did we kill it? Any thoughts would be appreciated.