Snapping at his own tail?

[swiftrat]

I just caught my male black wildtype doing a couple of spins in his tank, chasing his own tail and snapping at the tip of it. Can anyone tell me what this behaviour interprets as?

The water is clean and has a gentle airstone running. pH is 7.2 and he eats well. I don't see any sign of fungus infections or bugs on him, but still... any ideas as to whats irritating the tip of his tail?