Fighting rust colored growth on sandbed - may be related to RO/DI or city water

As per the title I've been patiently waiting for a nuisance to subside since February without any luck. Looks like diatoms and is stimulated by light, predominately on sandbed where the flow isn't as strong as on the rockwork. Rust in color, no strings or bubbles from it. More of an eyesore but I do have some sps that have lost color and I've lost a good portion of my orange digi colony which I've always considered to be fairly hardy.


Params are all stable but I don't test (or dose) magnesium, strontium or iodide and I don't have an accurate test for phosphates. I don't have outbreaks of other (green) algae which would indicate the phosphates are fine.


I have been watching my RO/DI over the summer. We moved into our house in June 2012 and I noticed, about the same time I started having issues, a similar rust colored discoloration in my RODI (RODI started around Christmas). I'm in a very old house (1929) in the SE side of CR, about 2 blocks from the CR Country Club and Cedar Rapids is supposed to have the best tasting water in the state but I wonder what else is in it. Last time I replaced the filters I had discoloration within 30 days with TDS near zero. Filters are 10 micron sediment, 5 micron carbon and 1 micron sediment before membrane and resin.


Although my output water has a TDS

 

Saddoris,
Thanks for your reply! I've spent some time looking for chloramine carbon blocks (interesting that last week's YouTube post from BRS was on this topic) and have been wanting to test for ammonia since this may be an issue.
My ammonia test kit is bad and I decided to test for what I could this morning: nitrates are in the upper 30's which is certainly a root cause of pale and dying corals. I'm planning substantial water changes this week to drop these closer to zero.
My assumption is the rise is due to my lack of maintenance on a DYI sock filter (felt) as I didn't read that chloramines in the RO/DI would produce nitrates - open to correction if I'm wrong in this thinking.
I do plan on getting carbon blocks designed for chloramines in the very near future.
-Eric