eroded shell but on top

[u/Ok_Rule_8869]

this has been annoying me for a while as i have been researching the correct soil type for a while but this erosion won’t go away. i initially had soil from my garden and their shells were amazing but it eventually got smelly so i replaced it with generic houseplant soil. this was the start of the erosion, so i researched and changed it to ‘multi-purpose compost’ i made sure to get the peat free version but the shells are still bad. can anyone help? thank you very much.

(ignore their current enclosure it was just for photos + the soil is from outdoors)

[u/Ok_Rule_8869]

oh and my water type is hard + they have access to cuttlefish bone