If there is just one filter it is probably designed to improve dirinking water by filering through charcoal.
Not what you want for your tank. There isn't that much different in price between ro/di or just ro. You'll want a filter that probably has at least 2 carteridges 1 carbon 1 filtering material, then an ro with a di on top.
This is probably one of your most important investments so you probably shouldn't go cheap here.
Do a search and you'll get plenty of recommendations. I use the Typhoon 3 and find it to be awesome.
Good Luck!