Well, that was fun...
In the picture above I noticed thread algae. According to this article
it could either be excess ammonia or organic waste. Ok - I have been trying to keep the tanks and filters pretty clean so I decided to test for ammonia. All 3 of my tanks tested 0.25ppm ammonia. WHAT?
So, that lead to a bunch of testing of my RO water, tap water, the distilled water I buy from the store that I fully trusted to be pure
(see where I am going here?) and the various equipment I use to collect water samples which I assumed
to be clean (never assume).
After many, many tests it was determined that my trusty sampling syringe (medical grade 10ml) was not as clean as I assumed. Also, the "high quality" distilled water I get from the store may not be as "quality" as I assumed.
Using proven - cleaned equipment I was able to consistently produce 0.0ppm ammonia tests of my RO water and tank water. Guess the end result is I need to keep my tanks / filters cleaner! Oh, and stop assuming!