Sand - ok but can be messy they still may eat some and sand can get gas build up, not so bad if gravel vacd or heavily planted
Rocks- ok but really need to be bigger than will fit in there mouth which isnt small, and imo a tank full of large pebbles is gonna collect all the muck
Sand under rocks would only make cleaning even harder and more chance of getting gas build up i would of thought.
I just use bare bottom tanks, with a couple of rocks and oxygen weed added when in breeding season, seems to work for me!