I don’t think that a serum is going to be enough as it only coats the surface of the hair. Using a good conditioner normally gets knots out and it is always best to combe the hair through with the conditoner still in your hair. The best detangling conditioner that I have used is from MATRIX and its called Moisture Cure (R155). This takes out all knots and conditions the hair without weighing it down.
MIRRORS hair lab