http://brs.li/NewCarbonBlocks When it comes to our tanks, what we're really trying to maintain is high quality, pristine water. That's why RO/DI water has become such a staple in the hobby for removing chlorine and other disinfectants as well as other impurities such as nitrate,phosphate and ammonia.
So what has happened in the last decade with disinfectants? First there has been a huge shift from chlorine as a standard disinfectant to chloramines, which are chlorine reacted with ammonia, to create a much more stable disinfectant compound. Since it is relatively new, chloramines are not well understood and there is a huge myth floating out there that a standard carbon block is an OK solution for chloramines. Amazingly almost no one in our hobby has actually tested this for themselves. I can tell you definitively right now that even high quality standard carbon blocks like this one are more or less a cylindrical waste of time and have no value treating for chloramines.
There are specialized chloramine carbon blocks which cost a few bucks more but will up to 20x as long, which is a pretty obvious value. It really makes no sense at all to use a standard carbon block now that these specialized blocks are widely available because they will save the average reefer money almost immediately, increase the quality of water they use and protect the more expensive membrane from the oxidizing effects chloramines have which will ruin the membrane.