depends on your demands for calcium. if you have 3 or 4 frags its really not needed. a good 5% watter change will do you good. but thats my opiniun.
to be hounest i have done a lot of research and have been contimplating one for a good part of the 6 months that have gone into building and planning my new 200g project. even though i left room for one, i dont know if i need one till i get fully stocked.