How far away is the new house from your existing house? It would not be unreasonable to include as a term of the sale of the house that the fish are your pets and not included, and you want to leave the fish where they are until you build a new pond and it is safe to move them.
I'm about 30 minutes away from my old house, This is an interesting idea that I didn't think of. I'd probably rather avoid this, though, as I don't want any terms that could jeopardize the sale of the house in this market.