So this applies both to my fresh and sw tank. I for the longest time was under the impression to use RO water because it was the purest the fishies could get. But while using it with the fresh, I noticed I could never balance the PH. The test would always read acidic. I did multiple water changes to help reduce ammonia but that still didn't put a damper on it. So I tested the unit itself, to do so I used 3 test tubes AND two different test kits (one for SW the other Fresh) so 6 tubes but same variables. (I currently have a BRS 5-stage ro system) In my test, I filled one tube with tap water, for the second I took off the final carbon block stage on the unit and ran the system and filled tube with that sample. Finally, I put it back on and ran it again. My results were as follows: tap water a good neutral 7 and both RO tubes tested out to be lower than 6 for both test kits. I understand you don't want anything like copper in the water for sensitive inverts. But am I missing something? I read up on it a little and found that RO systems essentially suck everything out of the water, but how do you get from that to neutral without costly buffering? I am sorry if the answer is in front of my nose and I am just not understanding it. x.x But thanks for words of wisdom.