I put the cuttle in my tank, and I have never had a change in my pH. The snails eat it, so I don't believe it's in there long enough to change it.
I assumed they were given to you like that, mine were like that too when I got them. I actually had one who's shell stopped growing, but it didn't.
And it had this big "V" cut up in front which would never grow in. The cuttle bone does wonders for them though and it doesn't take that long.
They seem to grow just like fingernails.
I soak the cuttle for about 3 minutes, and then stick it in a bowl and grind the end off with the end of a wooden or plastic spoon. It put about an 1/8 of a tsp in each tank.