Diatoms or cyano?

[Shilpan]

Hey guys! 100 gallon softie tank. Only a shrimp and hermit crab, my fish all died from velvet or Ich or brook and the tank is currently in fallow. I'm just taking my time to work on tank maintenance. I have this brown stuff on my sand. It was there when I had my fish. And now it's still there. My question is what's the cause of this brown stuff? Is it
a) poor water quality?
b) long lasting effect from overfeeding and it'll go away since I've stopped feeding (used to feed a cube a day)?
c) because I haven't changed my T5 bulbs in 11 months? Is this brown stuff characteristic for this?
d) because I rarely siphoned the sand since I let my sand sifter do it for me and so I wouldn't disturb the microfauna too much? And this is just build up of waste after 1yr and I need to change the sand and siphon more often in future?

There's no overgrowth of red cyano or green hair algae which I usually get when I don't maintain my tank. I have a little cyano on my glass because I haven't scrubbed the glass in 2 months since I was treating the fish. What's this brown stuff?


Params: nitrates 10ppm, phosphate: undetectable. I use chaeto, a skimmer, purigen and Seachem phosnet. And omg my microfauna has burst into life so many weird things crawling around without fish to eat them.

 

[lmforbis]

Probably not diatoms. They are a new tank issue. They require silica to grow and once they deplete what is there it is rare to have an issue.
My guess is Cyanobacteria. You can vacuum small sections every water change. You can increase flow. It is basically excess nutrients causing it.
There are chemical treatments like chemiclean which do work well but they don't fix the cause.

 

[Shilpan]

Ok thank you
That's easy to fix then since I have no fish just some regular maintenance should do the trick

 

[2quills]

Looks like a little cyano and dino. May want to get some new bulbs as well since you're past due. Cut back on the lighting schedule a bit while you do some manual removal etc. Might not be getting enough flow to keep detritus suspended in the water column long enough for your overflows to catch it all.