Might I say- I love this response! I have been studying my horse head loaches in a mixed substrate tank since this discussion started- my observations are humorous and tragic at the same time (this is my personal bedroom tank and has no relation to my research project in any way- sorry, just covering my butt
). My school of 4 is now a school of 3- I believe my largest one (5 inches) accidentally uncovered a fertilizer pocket under a piece of slate while twirling into the sand and pushing anything out of the way, apparently, that got in his way (his nose was bent and looking pretty tattered). I guess I didn't do as good of a job cleaning my substrate as I thought :-( but other than that tragedy the rest is comical. All my tanks are interconnected and maintained individually to support my personal curiosity about life's little 'brain turds'. Original preset water flow is completely neutral and water is conditioned (aged). 2X a day my system (based on a hydroponic garden 'ebb and flow' system) replaces 15% of water and vacuums/ processes waste gathered from filter for my aquaponic hybrid experiment) I know that seems way off topic but I think it may help a few people that are 'fish nazis' around here understand a little more about their hobby versus what it's TRUE ROLE in the broader spectrum of REAL LIFE APPLICATION. My balanced ecology allows me to move my fish from one tank to another with none of the real issues worried about in the normal home aquarium set up, so pardon me for forgetting ornamental aquarium rules and standard practices by not having enough time to research the biochemical breakdown and release processes for chemicals and their correlated biological impact on water chemistry yet- all I know is- my lettuce and rice are dying, hmmmm, looks like my water is a little acidic and my garden needs potassium, calcium and phosphorous. Ok then, this is my additive mixture- ADDED TO THE OUTGOING GROW MEDIUM FOR TERRESTRIAL PLANTS- the resulting water is filtered by the root systems of said plants, and the resulting filtered water is primed and conditioned for the next water cycle. This and more be topics in a rant I'll post in off topic to respect the OP and AA.
Back to my loach findings- they pushed all pebbles, gravel and small rocks to the outer edges of the tank and piled the sand high into a mound. Because I sold my soul and my 'thinks' lol, to the devil with deep pockets I am not allowed to show pictures until the patents aren't pending anymore, but once I can I have a picture of them atop their sandy mountain side by side by side, sifting for gobs of thawed frozen tubiiflex worms. That being said- in my observations sand is definitely appreciated, a necessity- not for kuhli, IMO for they don't have 'faces' they can get caught in between gravel or pinned by rocks. I'd say to just make sure your decorations (including substrate) are free of rough surfaces and jagged edges and maybe provide them some water sprite or other floating plant and some java moss to give them some comfort and keep them from feeling too exposed and satisfy their burrowing habit... On to the next part of my rant, lol.