Is it possible to have high organic mater in the water and no nitrates?

[Scuba Josh]

I have had mushrooms, (std red discs), die off one by one over the last month.I have little to no nitrates and perform weekly 20% water changes with RO water. I change the filter floss weekly as well as clean out the skimmer. Yet I have diatoms:mad2: The tank is running for 6 months now and I was diatom and algae free from months 2 1/2 -5. HELP ME PLEASE!
Is the die off creating the diatom bloom?