There are two things which might interfere with tall, tower style, cpu heatsinks such as the Hyper 212.
1. Memory modules with extra tall heatspreaders that sort of look like a comb. It is usually one of those memory modules in the memory slot closest to the cpu that will interfere with the heatsink. Standard height memory models will not intefere with the heatsinks.
2. The width of the pc case. Some cases are not wide enough. The Hyper 212+ will fit inside your Antec 900 case.