I purchased a Home/Reef unit from AWI a couple weeks ago and am at my wits end on how to test the TDS. AWI assures me that the TDS will be at Zero on the DI side. However, I get the same basic reading on both the RO and the DI. I got a permeate pump and installed it last night and also replaced the Horizontal DI cartridge. But I still get 15 PPM on the RO and around 20 on the DI. I have a new tester that I am using. It is a HM Digital TDS-3. Any thoughts? Thanks.