I use RO for my tanks and TDS is under 10 last I checked. My 240 FOWLR shows no signs of problems. Parameters are great, no algae problems and fish are doing well. I will eventually upgrade to an RODI unit, but for now, this RO unit that came with my setup is doing okay.
I agree that zero is ideal, but I don't think that FOWLR is as demanding when it comes to pristine water. I'm actually using it for my 60 reef too and beyond algae, there seem to be no big issues. Fish and corals have all done great 14 months in.
Sent from my iPhone using Aquarium Advice