I use alfalfa pellets for Dudley's dig box. Other people have used millet seed with good results too.
I don't think a small area of sand would hurt them though, if you wanted to try that. Make sure you wash the sand so that it isn't dusty, and keep an eye on them to make sure they don't eat it. And don't get calcium sand.
I just set Swordtail's timer for his bath and paused it so I could actually fill his soaking bowl up and he crawled over my phone and canceled the timer
Mirage came out of brumation on April 26. He was doing great. On May 2 he started acting funny. We just redid his tank, and he keeps going into one of his hides. He just lays there. He shows no intrest in food. HELP!