Bettas keep dying

Question
HI,

I have brought three bettas now (i was told there really easy to keep) and after a couple of days they stay at the top in the same corner, start losing their colour (go very white but its not a fuzz it's just loss of colour) then when i wake up it's DEAD, there my favourite fish and i want to trying again but am i doing something wrong?

i have a 27 uk gallon tank (120 litres) with 4 cherry barbs, 2 honey gouarimes, a red tailed shark and 3 glowlight tetras and 2 mollies. Have had the tank 3 weeks but it was used before along with the filter (ehiem2213) and nitrites read 0. have a pump to increase oxygen in the tank, with a large ship and a few faux plants in. water is kept at 26 degrees centigrade.

The other fish are doing great i feed them all on flake food 5/7 days, blood worms once a week skipping one day aweek without any food to encurage them to look for food around the tank.

Answer
Luke.

Well, the only problem I see is you trying to add a Betta to a tank that is housing Gouramis. Both of these fish are called labyrinth fish. This is what allows the fish to breathe air like we do. You cannot keep two different types of fish like this in the same tank. The Gourami's are most likely killing the Betta. Also, when a tank is over populated, it will cause the fish to start fighting. As long as you have the Gourami's you will not be able to have a Betta.