why did my fighter die

Question
i had a fighter fish which was with us for only few months we got transferred n carried him with us he had lot of water n habitat changes in a short span after some time he stopped feeding and laid at bottom in a twisted position always with its head down he was hyperventilating all the time n appeared much paler i euthanised him after waiting for a week for him to improve but it didnt m still confused what went wrong we have one more female fighter bought with him only but who is still doing great under same circumstances what went wrong with the male was it not able to take the stress?m scared i euthanised him without knowing what was happening

Answer
Hello
Never put a male betta in the same tank as a female betta.  They will fight, causing injury, and death.

If he was alone in a tank he most likely had Swim Bladder Disease.  Swim Bladder Disease is caused by poor diet.  Poor diet leads to constipation, and if not treated quickly will lead to Swim Bladder Disease.
 
I will give you some information on the Betta Fish, and how they should be treated, male, and female.

A betta needs at least 2 gallons of water, but in my book 5 gallons is best.  
In his habitat the betta may live in shallow waters, but he has lots of room to swim.  
Keeping Bettas in small bowls, containers, vases etc, is cruel.  The Betta is one of the most mistreated fish on the market.
 He needs a heated tank,  Temperature should be 78-80 degrees.  He is a tropical fish, and is cold under this temperature.  Cold bettas get sick, are miserable, and live unhappy lives.  They should have a silk plant in their tanks, so that they can rest on the leaves  They also need a little tunnel or some knick knack that they can go through.   You may add 1 teaspoon of aquarium salt in his water which has been diluted before adding to his tank.  1 teaspoon to 5 gallons of water.
He needs a varied diet. This is very important.  Betta pellets one day, Betta Flakes the next, Daphnia the next, and bloodworms should be given once a week as a treat.
He should be given one frozen pea a week, and on the day you give him the pea, he should fast.  
Cook a frozen pea until it is well cooked and tender.  Remove the outer layer, and cut into tiny pieces.  
Feed your Betta one piece at a time making sure it doesn't fall to the bottom of the tank.  This helps to clean out his system.
A betta is prone to constipation, and constipation leads to Swim Bladder Disease.
He should have 2-3 small meals a day, and not one big meal.  We must remember that his tummy is the size of his eye.
 Example:  2 pellets for one meal.
Water changes are very important.  Water changes must be made every week without fail with a good water conditioner such as Aqua Safe or Stress Coat.  There is also a conditioner made especially for bettas.
When treated well, a betta can live up to 8 years, and be a very good friend.  
He's an intelligent little fish, and should be teated with kindness.
Too many vendors know nothing about the betta, and don't care...so don't give good information on how to keep them.
 They are there to sell, and will say anything just to make a buck.  
This turns my stomach over.  Unfortunately there are no laws to protect fish.   
Bettas kept in small bowls, and tanks, will get sick, and are very hard to cure in such little water.  They end up dying, and
that hurts the person who has grown so attached to them.  They don't deserve to be treated this way.
I just thought I would tell you a little on our friend the Betta.  If you have more questions do not hesitate to write me.  I have bred them, love them, and know what is good for them.
Hope this helps
Lynda