In Sri Lanka there is a big controversy about dua after the Fardh Salaat.

Answer

The only correct method is the Sunnah. The Sunnat method of the Dua after the Fardh Salaat is a silent dua. Everyone should make his own Dua. Those who are making the long, loud dua are following custom – bid'ah custom which has become entrenched, hence they refuse to abandon it. It is Waajib to abandon the bid'ah custom and adopt the Sunnah. If the entire Sri Lanka is embroiled in a bid'ah, it does not follow that the few Muslims who have understood the Sunnah and practising the Sunnah should follow the majority.