Anyway. I would not count out Tap water or failing RO cans either.. Municipalities change things and tell nobody. As long as water tests as potable, they will make changes. Carbon filters in RO systems fail a lot faster when some changes get made.
This would be my first stop as well. About a month ago I noticed I was going through DI a lot faster than normal. TDS came out 0-1 but it led me to do some investigating. I Phoned Spectrapure (since they are in my backyard, they asked me to bring my whole system in. Come to find out my carbon block was spent and my membrane wasn't working to capacity. I bought all new prefilters, new membrane and added a silica buster and maxcap to the end of my DI.
Over the course of 3 weeks of large water changes (total of 250g for a 350g system), I have seen a huge improvement in coral color and PE.
I don't use Fritz, I switched to Red Sea blue bucket about 6 months ago from IO. Here's the good news for me, I sent in an ICP test right before I started the large water changes, and I will send in another in about 4 weeks. I hope to see a dramatic improvement between the 2.
Just to piggy back, @Crabs McJones check for chlorine in your brine line of your membrane housing, it should come back undetectable.
And I personally like the ATI ICP test because it also allows you to test your RODI water to rule out elevations of elements from salt.