At the risk of sounding like I'm pushing the barrow, - and I'm not - The filter combo you may want/need will depend on your water quality and what you want to change with it. Some of the above is not entirely correct.
It gets pretty complex, there are RO, DI, systems, different filters etc. You can make it super pure, which can actually be too pure and no good for drinking etc. Depends also on where you are starting, and where you want to go.
I was in the coral world for a few years, and water is a big thing, but being on pretty good tank water I never really had to go to filter systems.
You can give psi (think its Pete?) a call and he will chat and sort out whatever it is you want. I think his prices are pretty good, but you can always shop around.