HELP, fish keep dying in tropical tank?

Help my poor fish keep dying.I have a 200lt tropical tank set up with the required heating and filtration system.It houses 6 zebra dandeos, 2 barbs, 4 guppies& 2 gouramies. I use aquarium salt and do a third water change every two weeks using water conditioner every time.However my fish keep dying at what is an alarming rate.(At least one a week) They have a healthy varied diet of flakes an live food.I have noticed a day before they die there scales develop this dull sheen and they quieten right down. Please help my poor fishies?