How would you go about replicating something like this?

[Freezekougra]

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Look at 0:30 or so, that kind of "substrate" would look real unique in a fish tank :O

 

[Danger_Chicken]

That's basically mud and debris. It's possible to replicate but would be a huge headache and require small fish that wouldn't dig or upset the substrate.

 

[packer43064]

^^^
Agreed. Thinks play sand times times 20. It would look like muck always.