Well toxic pretty much defines itself, walnut shells are toxic from a natural chemical known as juglone. plus beardies cannot digest the shells if by mistake they eat it( which they usually do) it wont pass threw them good and could case serious problems , i don't trust most substrates like calcium sand and other lose dirt like mulch crap. yes i know they are desert animals and live on sand but breadies in the wild wouldnt last as long as our beloved house pets due to the healthy environments we have them in. use solid flooring, i use cardboard. (i just either get used cardboard from work or behind any restaurant or pizzeria. every place usually has a dumpster only for cardboard.) i use flat pizza boxes there perfect to cut out and shape. tiles are best because you can take out each one and clean off poopy or whatever (im going to update to tiles soon) . i dont really trust the reptile carpet i've heard of the maternal coming apart and the toes of beardies getting stuck, thus making no blood circulate and the breadies toes fall off. just my two cents everybody likes there own stuff