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Thanks for all your advice. Clanad, Its a Border terrier btw.
She has 2 x 15 minutes walk per day, less in this hot weather if she flags, and we take penty of water that she has @ 5 minute intervals. Inbetween walks, she has house exercise, and lots of play. She has only just started the tail chasing and of course i only want whats best for her, which is why i asked advice. Im an experienced dog owner, (sadly i recently had to put our 15 year old Westie to sleep) but tail chasing is not something ive personally experienced with any of my previous dogs, so i just wanted to be clear on the psycholocical issues, and nip it in the bud if it was advised to be a negative behaviour.