The compatible breeding partners with Goldeen that can teach it Psybeam through breeding would be Remoraid and Octillery, which both learn the move at level 22. Simply breed a female Seaking with a male Octillery/Remoraid with Psybeam, and you should get a Goldeen with the egg move you want.
Though, Remoraid and/or Octillery must be traded over from Emerald, as they are unobtainable otherwise. Remoraid and Octillery can be caught in Emerald at the Safari Zone in the Southern Extension, both both of which can be caught with the Super Rod likely knowing Psybeam already, and Remoraid can also be caught in that location with the Good Rod.
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