If using an RODI unit- hook it up after your water softner. This is the recommended approach by Bulf Reef Supply and most others. I used to have my RODI unit plumbed before my water softner. I was blowing through membranes consistently in under 12 months. Remember, you will still need to adjust both GH (calcium, magnesium) and KH (baking soda) for your RODI water.