as I read today on Reb Lazer Brody's blog, http://lazerbrody.typepad.com
so I would like to quote from Rav Neuwirth's book, Shemirath Shabbat Kehilchathah, in his memory, may it be for a blessing.
Here he talks about tying and untying knots on Shabbath and Yom Tov. Tzitzit fall under the category of knots which one is not allowed to tie on Shabbat or Yom Tov which must not be tightened if they become loose.
Vol. 1, Chapter 15
50. c. As a result, one should beware of tightening knots of tzitzith which have loosened.