I agree with Alicia. Consider mixing a little cypress mulch in with the aspen. Maybe a 40% mix of the 100% cypress mulch. Either the Zoo MED brand or the "all living things" brand. Let them kind of acclimate to the substrate change, maybe skip the next scheduled feeding then pick it up after that. Don't get any cypress mulch from home depot. You can get the right one from petco, petsmart right away. Good luck.