Brown dusty layer on all plant leaves
Hi all, I am newbie planted tank hobbyist. It's been 2 weeks since I've setup a 65 lit (~17 gallon) low tech planted tank with Bacopa, Amazon Sword, Ludwigia, Bolbitis and Anubias. I've used Fluval Stratum as Substrate and I'm seeing a lot of brownish black dusty layer on all the leaves (please find the image attached).
I've done couple of 30% water changes and gently washed off the dirt layer on the leaves with fingers. Annoyingly, the dust keeps coming back by the next day. At this point, I'm not even sure whether it is dirt from Substrate or Algae outbreak (Brown Algae/Diatoms). I've also observed Amazon Sword's leaves turning yellow and Bolbitis leaves turning black.
I have 21W LED light (turned on 8 hours a day) and started using Seachem flourish for past couple of days. I haven't observed any major spikes in Ammonia/Nitrates/Nitrites until now.
FYI: No CO2 setup. No root tabs.
I really appreciate your thoughts on this.