Digging holes

So another quick question, and something I've noticed recently, in the current 20l tank that my 2 fantails are living in while their 110l home finishes it's cycle, but the larger of the 2 has begun digging holes, like pointing straight down and moving the gravel out the way, does this for 10 seconds or so, and then moves on, is this just relatively normal and its keeping itself amused? Or is it something I should keep any eye, i know it won't be able to do it in the new tank as that's covered with sand on the bottom