I have one that consistently sits under the filter, too! But after rearing axies for some time, I found a little gizmo they really enjoy: the bubble bed! Imagine a large, flat bubble stone that you place in a corner of the tank, under some of the sand- there isn't a minute in the day that one of them isn't lying on it and getting their massage. I've installed a second one in the males tank so they don't feel left out.