what supplement should I take to strengthen my fingernails.

Although not generally marketed as a solution for your issue, I found that taking a broad-spectrum amino acid supplement hardened my finger nails, caused my hair to grow faster (including my whiskers) and boosted my hemoglobin rating when I donate blood even though I stopped taking a ferrous oxide pill. I am fond of Amino 1000 from GNC. You'll find it at

http://www.gnc.com/product/index.jsp?productId=2133702