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Dark Blanket...algae or diatoms?

Subject: Dark Blanket...algae or diatoms?
by Philosophico on 10/2/2019 19:23:30

Hi all,
Looking for some help and advice with a stubborn diatom/algae problem.

I have a dark mat of 'stuff' that insists on growing over the piece of oak in my tank and occasionally forms in mats on the gravel substrate.
Tank is 200 litres, moderate planting, light fishload, pH controller with CO2, CO2 is at or near 30ppm (tap water is pH6.9 and the controller aims for pH6.0), using EI ferts and weekly 50% water changes, lighting is programmable LED (low output, 8 hour photoperiod).
Am=0 Nitrite=0 Nitrate =15ppm, Phosphates=5ppm, GH and KH I have measured but can't remember....D'oh!.....if memory is correct GH is 8 and KH is 2.

The 'stuff' can be peeled off the wood during cleaning, feels a little slimy, but seems to break down into a 'wet dust' when rubbed between the fingers.

I've scanned the web for pictures and descriptions of types of algae but can't see anything resembles the gunk in my tank.
The nerite snails (four in the tank) seem to travel over and around it but can't keep on top of it (if they are in fact eating it).

Any insight into what this stuff is would be greatly appreciated.