At first thank you all for your kind replies. 🙂
I will definitely keep this thread updated since the outcome might help others as well in the long term.
Behavior didn't change much, except they are reaching the bottom glass and the pile in the middle is already as tall as my hand width. 😅
Sounds fair, I'll keep an eye on it. The confusing thing is, they pretty much ignore the "Tabis" krill food tablets but keep up digging.
Not really. It is basically one spot only. Might be a vortex in front of some driftwood there. But the line of algae marks the initial sand height.
I'll watch that, thanks. (It was a mix of 2/3 fine sand and 1/3 small gravel below - they just swooshed all the fine sand away 😅)
I already called the manufacturer of my new aquarium to make it a bit taller so I can increase the sand layer... just in case 👀