INT ON DEBIT

What is this INT ON DEBIT that makes my account fees sky rocket every month. Is this a trick to get more money from my account? FNB is crazy with fees and they do it in a corny way coz its all the fees they never told u about. For some reason when u swipe, transactions do not got thru even though u have money in the account and 3 days later they go thru and by that time u have already withdrew cash and this leaves ur balance to be negative and then they send u messages about u exceeding ur limit ****. Something is definitely happening with their banking system that is underhand, FSB or any other relevant authority must be notified about this, I think starting an NGO that investigates these financial institutions would be a great idea.