betta has fit dying

Question
5 gallon tank One betta three heterandria formosa nO FILTER REGULAR WATER CHANGES., hEATED. bETTA LOOKED AND ACTED HEALTHY. gRETTED ME. aTE WELL. sHOWED HIM MIRROR HE FLARED AT IT  LIKE OTHER TIMES NO PROBLEMS. sUDENLY HE BAGAN TO HAVE A FIT. sWIMMING WILDLY AROUND TAN. sOON SETTLED DOWN SOMEWHAT. sWIM FAST TO ONE SPOT STAY THREE SHORT TOME MOVE FAST TO ANOTHER, oVER AND OVER AGAIN. pUT LIGHT IN HIM SAW GOLD COLORING AROUND GILLS DUST ON BODY ONLY SEE WITH LIGHT. sUNDAY NO TIME TO MEDICATE. eVENTUALY STAYED AT HIS FAVORITE SPOT. bARELY RESPONDING TO STIMULI. eNDED SUFFERING. wHAT COULD HAPPEN SO SUDDENLY.

Answer
There could have been a lot of things to have caused this. What was the temp. in the tank? Showing the betta a mirror made him think there was another betta and instinctually wanted to fight with the other betta. Could have been a fungus or bacterial infection on his gills. A filter is very important to a successful aquarium. Sorry to hear he did not make it, I wish I could be of more help but there really was not a lot of information to go on. Happy fish keeping!!