For tomato-based dishes a pinch of sugar may help (most Italians use a pinch of sugar in all of their tomato sauces to balance the acidity). And by a pinch, I mean literally, a pinch. Less than 1/4 teaspoon. Start small, taste, taste, taste, and see what it takes to get your dish to where you want it,

What type of dish is it? What other ingredients are involved? Thank you. ;o)

Sometimes a bit of water can remedy the situation.

Or add oil to round out the flavors. Creams, cheese, milk can also soften the too acidic dish/dressing, etc.