Mix half a cup of vinegar, 1 tablespoon of fish sauce or patis and crushed garlic on a deep dish or a large freezer bag.
Press or crush down the garlic to release more garlic flavour into the mixture.
Set aside about a tablespoon of the marinade mixture in a small bag and add in the garlic slices so that it absorbs the vinegar mixture flavour.
Put the milkfish into the marinade mixture. Close the bag and refrigerate it overnight.