After catching or killing or running away from the red gyrados it will say u have obtained the red scale. Got to Mr.pokemon house and talk to him and trade it for an exp share. The red scale has no use anyway that i know of
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how to get exp.share in pokemon blue
first go to Mr. Pokemon first have the red scale and he will give you it
In Castelia city.. Is in one of the buildings last floor
no you can trade soulsilver pokemon to black
Talk to Mr.Pokemon after you get a red scale from the red gyarados