Getting a water filter do I need DI water?

I am looking g at a water filter and I can get one for RO water what is what I think what I out in my tank now as I am going to a shop and getting my water from there. It is showing I can get a Deionizer to make DI water and looking on the internet it looks like everyone is using RO/Di water? So is it best I spend a bit more and get it or is it not worth it? Here is a link to the filter I am looking at getting.
http://ift.tt/1QHGoYk