Hydrogen sulfide stain on background?

[mynameisnobody]

Hello all, Because I will be moving, I decided to rehome some of my stock, but definitely not all. I sold off my shellie colony and as I cleaned out the sand, I noticed that the shellies built up a little sand mountain on the background. When I removed that area of sand, the background appeared stained black, just like some of the deeper sand itself. Question is, now that they are gone and most of the sand is gone, will the black stain on the background go away. I believe it’s a hydrogen sulfide stain. Do I need to scrub it to get it off or will snails eat away at it little by little? Does the water flow help alleviate the issue? Any and all feedback is appreciated. 
ps I can post a photo tomorrow as I didn’t have my phone near me when my arms were wet. I’ve lined some ez planters in front of it to make it not so noticeable, however tomorrow I will be doing a water change so I can snap a photo. Thanks again 

Edited April 12, 2021 by Manny

[DShelton]

Please post a photo. Does the stain go all way to the top edge of where the sand was and is the bottom of the tank black?
 
We can detect H2S in very low concentrations like 50 - 100 ppb. You would have known if you had H2S as you could have smelled it. 

Thinking about the chemistry, there is a pretty small chance you had enough sulfur (unless you were dosing it sulfate for some reason) to produce H2S in enough quantities to detect, let alone possibly stain the glass.  Your sand bed would have to be extremely deep, say 8 or more inches to possibly create anaerobic conditions suitable for the sulfate reducing bacteria to thrive. 
 

 

[mynameisnobody]

Ok folks. As promised here are some photos. Funny thing is that it was jet black yesterday and today it seems the water flow has cleaned it up a bit so I think I have my solution. Please pardon the shotty photos, I stink with a camera. Thank you kindly

[RockMongler]

Its probably some flavor of low light algae, fungus, or bacteria.  Exposing it to light is probably doing all the work of getting rid of it for you.  Before, it was probably the right mixture of nutrients and light, and now that you have disturbed it, it's probably going to shrink back and not be much of anything.

You'd probably know if you had any significant amount of hydrogen sulfide.  The human nose is very sensitive to it even at low concentrations (IIRC, ~ 5 ppm is enough for your nose to get that very specific rotten egg smell, and your nose burns out to being able to sense it ~12 ppm, again if I remember some of my safety training materials).

[mynameisnobody]

@DShelton @RockMongler 

yes I forgot to mention, while removing the sand it stunk to high heaven. Now everything is good and the smell is gone, but it’s smelled exactly like rotten eggs. My question wasn’t as much what is it, but more how to get rid of the black stain on the background. Thanks again

[RockMongler]

If it doesn't continue to clear up on its own, I'd say clean it with a brush like like you might use for cleaning dishes.  That should be able to get into any nooks and crannies and pull any gunk in there out, assuming the light and snails don't do it on its own. With the pictures you posted, it honestly isn't super obvious (but as I'm sure we all know, tank photography is hard and capturing what you actually see can be tricky).  If you hadn't said there was a stain, I probably would have just attributed it to lighting/texture.