Best thing for strengthening nails is natural ones of corse.
My advice is: Olive oil and lemon,use about 4cm of olive oil and squeeze one half of lemon,put it in some small pot and store it in fridge(it can be used for few weeks).Before use put it on cooker and make it hot(not too much,you`ll see what temperature is suitable for you) when it`s hot put your fingers in oil and lemon trough intermediate phalanges and keep it in till oil is cold(about 10minutes).It is important to do that almost every day. After some reasonable time you`ll have strong and flexible nails(i noticed result after 2weeks).
I use that recipe long time ago (6-7 years),and it helped me permanently,since than i don`t have any problems with nails,they are strong and flexible what is most important.
I know it is a little bit complicated to bathe your fingers in oil but it`s worth it.
P.S. After you finished don`t wash hands immediately,give them good wiping with paper and prolong washing as long as you can (30-60minutes).