New to darts, so please forgive me if this is a stupid question. We have a tank of 3 Cobalts and a tank of 5 Auratus Super Blues. All are probably 6-9 months old. The last couple of days I've noticed one of the Cobalts sitting in the hollow of a stump, butt up against the stump, and rubbing its legs against the stump and then itself. Today, I saw an Auratus doing the same thing, but against the glass instead of a stump. What are they doing? Is this normal behaviour?