Coralline Algae only on Snail Shells

[Soren]

What does it mean if my FOWLR is growing coralline algae on snail shells (occupied or empty) but not on the rocks or glass?
I have noticed spots of what appears to be dark red coralline algae growing on snail shells (mostly empty ones).
There also appears to be patches of purple and lavender coralline algae growing on the shells of larger live snails.
As far as I can see, the rocks and glass are completely devoid of coralline algae, though one rock that was re-situated seems to have coralline algae on the bottom where the sand previously covered it. I am not aware of any organisms in this tank that would consume coralline algae.

At this point, I do not have any pictures to share, but I will try to get some soon.
Water parameters seem to be pretty stable, though I do have high nitrates right now that I am working to correct with water changes after acquiring this tank about 4 weeks ago.
Salinity: 1.023-1.025 specific gravity
pH: 8.2-8.3
Ammonium: 0 ppm
Nitrites: 0 ppm
Nitrates: 200 ppm
Alkalinity and Calcium are both maxed out on the color-changing test strips.

 

[KyRo]

Your snails probably had the coralline on their shells from where you bought them. That's usually how it gets introduced to tanks. I kind of have the same thing and I just think it will take time. If you want to speed it up a bit, maybe gently scrape the snail shells so you have some particles that can seed onto rocks.

 

[sfin52]

Grab some corlaline from a trusted source. Take scrapings mix with sw in a blender pour into tank.

 

[Jib]

You can buy some bottled from Algae Barn to help speed it up.

 

[Soren]

Your snails probably had the coralline on their shells from where you bought them. That's usually how it gets introduced to tanks. I kind of have the same thing and I just think it will take time. If you want to speed it up a bit, maybe gently scrape the snail shells so you have some particles that can seed onto rocks.

Thanks for information, KyRo.
The snails and empty shells came with the tank when I got it a month ago. They have been in the tank for a while, so I find it strange that the coralline algae is on them but not on the rocks. I have noticed it growing on the shells from next to nothing (though I did not take careful stock of coralline at the start). I did add some CuC about 2 weeks ago which might have seeded the coralline?
I like the suggestion of scraping to seed rocks. There is no rush, though, so I may just see what happens over time. I do not have issues with other algae right now.

Grab some corlaline from a trusted source. Take scrapings mix with sw in a blender pour into tank.

Thanks for the suggestion, sfin52. I just got a bottle of Purple Helix(R) purple coralline algae that I was planning to use for my new setup, but realized that it expires quickly as a live culture, so I may be using it in my existing FOWLR to seed on rocks.

You can buy some bottled from Algae Barn to help speed it up.

Thanks for the suggestion, Jib. My question is more about why it would grow on shells and not on rocks, though it seems likely that it has not been seeded in the tank and may have been indirectly seeded on the shells before entering my tank.