The "DI" is the final component in the RO system. Not required for a reef tank but recommended for sure. You will not want DI for your drinking water, it makes it real bad. Don't need to spend a lot of money on an expensive unit, I have spend $150 and $850 (back in the day) for an RO unit, they provide the same results. I use a modified 100GPD aquasafe unit to protect my reef.
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I'm out. 
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