Sand Changing Colour

A question that I hope you guys n gals might be able to help with... 

My sand substrate started life as white sand, but over time it is getting darker and darker with distinct levels of colour difference going on, darker at the top and fading down to nearer the original colour... 

The majority of the tank is pea gravel over a sub layer of John Innes #3 and I guess that's likely to have a large part in this equation, but bottom line is that I don't know...  I see other folks' pictures with white or pale sand and they look to retain much of the original colour, so I'm a little curious and possibly even worried about what's happening with mine - especially as it might be something that needs addressing that I don't know about...?

This picture is when the tank was still very much in its infancy, but you can still start to see the change: 



This pic is more recent, but it has gone darker still since this: 



Do I need to be worried and/or is there something I should be doing about it...?   

I agree with daveyng, the dark next to the glass does look like algae. The edge of the sand is exposed to the light which will encourage algae to grow. i get that in my betta's tank which does not have a trim at the front so the whole depth of sand is exposed to light.


Dark patches will also occur under decor. These patches are due to anaerobic bacteria, an integral part of the tank's ecosystem. They grow under decor as water does not circulate well and becomes oxygen depleted - and anaerobic means 'no air'

I don't think there's any way of getting rid of it. I just go along the edge with a  plastic scraper pushing it down between the substrate and glass to remove the algae/film when it gets unsightly.

Thanks for the advice guys...  For the now, I'll leave it be I think...  Definitely don't wanna mess the tank up - might arrange some sort of temporary cover so as to deprive that layer of light, maybe... 

I do have a great tool for the uncovered glass, so that's not a problem as such:  https://www.amazon.co.uk/Tetra-Scraper-Floating-Handle-Aquarium/dp/B000S5JMXI/ 

But as you guys say, it's beneath the sand where the tricky part is... 

I do vaguely recall reading once about a snail type that's supposed to be good for finding that layer of algae and will consume it...  Probably Nerites at a guess...?  My old memory could just as easily be playing tricks on me though...

That's the one labelled: "#1.JPG" presumably Sue...?  I can't tell either - looks Nerite shaped but not colouring...  I'll have to investigate more I think, as the temptation for a single Nerite is definitely there - it is the black ones I like, forgotten their proper name though... 

I don't know if the snail in question is actually a nerite or not. it is the one in the first photo in this thread https://forums.thinkfish.co.uk/gallery-showcase/snail-photos/

I recognise that possibly-nerite's colouring from being similar to https://www.pro-shrimp.co.uk/snails/1558-horned-nerite-snail-sun-clithon-corona.html. My nerite spends time in the location you and Sue describe - but I don't know whether or not he eats the algae, for the same reasons as Sue mentions.

If the places you cannot see have lots of plants or other decor the snail will be able to turn itself back over. I would say they need something to grab hold of up to an inch away from the edge of the shell. If you have spots wider than 2 inches across with no plant etc, it may have trouble, though that increases to 3 inches for big snails as they they are about an inch across.

Well, it's official...  1 x Black Helmet Nerite Snail (Neritina pulligera) has been ordered...!!!

Chatting with a friend of mine earlier today and he was of the view that Nerites can right themselves if given enough time...  So much so, that he no longer corrects his...  Not 100% convinced that I'll have that same level of trust that they will do, but we shall see... 

Definitely a good chance of that happening with the four sizeable Corydoras that I have, who all tend to blunder about knowing that whatever gets in their path will move or be moved, lol...