cypress mulch is fine. even coco fibers and such can cause impactions. a small amount of earthy matter shouldnt harm your frog if its healthy. it will pass through. i adopted a marine toad that was kept on bed a beast wood chips or something and after having it a few days it crapped out a couple chips. i do agree that for the pixies and such it might not be good cause they like to dig down into the substrate. after using cypress for years with my whites treefrogs i switched them and my marine toad to a peat moss/top soil mix. and added some sphaygnum moss to the top for looks. i use live plants in my treefrog tank with this and the toad loved digging into it. plus its a helluva lot cheaper then buying pet store brand name substrate. i also use branches from outside. i just rinse them under hot water and let them dry in the sun.