I would highly recommend a RO/DI unit. It plays such a role in good water quality-and thats our #1 goal in reefkeeping. HIGHEST WATER QUALITY possible!
I am still new myself (3+years) and my RO/DI unit is the best money spent so far to help alleviate my water problems.
When I started, I was treating tap water and had no tank problems at first, then I started adding more bioload to my tank and the algea problems started. After help from many on RC, I realized I was adding un-needed phosphates to my tank and feeding the algea from using tap water. Once I got my RO/DI unit, the algea disappeared. I feel its well worth the money and would really suggest getting one.
Hope this helps
jolene