It depends on the type of fish and if we sell them and need them Normally it only a small Store credit
They will trade if they have room to house your fish.
I've seen Steve do it before. Both times the person was breeding fish that Steve had a market for, and they had met before. So, it was a bunch of similar fish, not just random trade-ins.
You can buy yes, but trading I'm not sure of. You should call them
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