I’ve heard people say it’s better to make a knot at the end of the Tzitzit so that it doesn’t unravel. But I’ve heard other people say it might be a problem of Taase Velo Min Heasui. What does the Halacha say of this?

Since the knot is on each string individually, and it's purpose is to prevent the unraveling of the strings there is no Taase Velo Min Haasui issue, neither is there a problem of adding knots over the number our sages ordered us. Hence, there is no problem at all. Rabbi Dov Lior