Issues would be that you should first see if it's allowed. Next the size of the tank may need to be smaller, as apartment can be on the smaller side in Manhattan. also the flooring needs to be able to support the weight, so you may need to have a smaller size tank because of it. Some people also have issues with the sump, as not only the additional weight there is the possibly of overflowing and flooding the place, plus the return pump could cause varibrassions issues, which people below may complain about it. Making RO/DI water could be a problem, as you may not be able to plumb it into your water lines, plus because of space you mayn't be able to store the water, this may cause you to buy water at LFS. Also you may have heat issues, as some place can be on the hot side. Outside of that the running and equipment is fairly the same.