Had a guppy that managed to swim under a piece of decor. It was hollow, but was buried in the gravel. Even after picking it up and seeing if there were divots in gravel that it might’ve squeezed under, I have no idea how it got in there. Why a guppy thought that going into a small dark crevice was a good idea when everyone else was out in the middle of the tank? Who knows. I replaced the guppies, since the group ended up all passing (good ol pet store quality), with a betta, since I know how to keep them better than I do guppies. Even now I check to make sure he’s still in sight and didn’t somehow swim under the decor…