Betta males DEAD!!

Question
I'm heartbroken, our male bettas are DEAD. We have two 10gal tanks which normally contain 1 male and 2 females in each. Our first male, Ninja, survived in a filtered tank for 2wks (we had him for about 5wks total). Next to die was Maverick our first crowntail. Our last two males, Ninja2 and Juggalo (who was a crowntail) survived for 6days. We has them in separate tanks of course (considering both were males). We've had the water from both tanks tested and everything came out normal. I'm just wondering what we are doing wrong??? Why can't we keep our males alive??? We've had 2 females (Coka and Crystal) die because they thought the filter was their friend and their fins got stuck. After that their tank was cleaned. Our other females, Sally, Ruby and Rainbow are doing GREAT!! As you can see I'm not considered about the females considering their deaths were accidental but I fear replacing our two males. Please help!!!

Answer
There could be alot of reasons why your bettas are dying,purely coincidence?, stress, the supplier? if all from the same breeder?, try putting a little bit of the aquarium salt in the water, and the ph level should be just a little bit acidic, also if you have live plants in there they could be poisonous for bettas so try using plastic plants, in the winter the water can get fairly cold if you dont have a heater, try and keep them in a sunny spot during the day.