I lost a red cap fish... please help me understand the reason...?

Once you said the word bowl the reason is very clear. No fish can live in a bowl, especially not large messy fish like goldfish. He would have needed a minimum of 30 gallons with double rated filtration to live out its full life span. In a bowl ammonia builds up quickly and poisons and burns your fish. It probably died from growth stunting though, since its body didn't have room to grow to its full size (should be over 8 inches) and its organs kept growing until there's no more room and they shut down.