Vale! was kind enough to take a look down a microscope. (Here)KevB wrote: ↑Thu Jan 21, 2021 20:23 pm If it's diatoms then I even get that in my big tank with external filters but in my case I think it's down to the hard water. Yours is softer so I wouldnt expect you to have diatoms in either of yours much, it's an odd one. Could you get a scrape sample and get someone to look at it down a microscope, at least you would know if it was diatoms, plant or fungal so you could plan how to tackle it.
His initial results are green algae, diatoms, cyanobacteria and some mystery organisms (little creatures).
The black stuff has been building up in to smooth layers which I scraped off today.
I can only describe them as being like thin layers of paint, very smooth.