Chloramine is a relatively new problem for water. Most manufacturers, including shower manufacturers have not mastered it yet. We do have the whole house solution. You will need the 20 inch big blue size because a smaller size will reduce your flow rate too drastically.
Here is the housing you will need. (you will need the bracket and wrench also)
http://www.waterfilters.net/culligan/wholehouse/housings/20_BigBlue.htm
Here is the cartridge you will need:
http://www.waterfilters.net/Pentek/wholehouse/Specialty_Filters/CRFC-20BB-Filter.htm
Thank you,
WaterFilters.NET