If wishes were fishes, we'd all eat like kings. Unless of course you don't like fishes, and then that's another kettle of fish, though I imagine still not to your enjoyment. So I would assume you'd be eating exactly the same as you do now, because who's going to force you to eat the fish? Certainly not me. I'm not up for the effort. Although as far as I know, kings didn't eat a lot of fish anyway. Maybe it just means we'll have crazy large portions because some kings were obsessed with having lots just to show off their wealth and power.
Also what would we do with all those fishes. You can only eat so much fish. We could make dog and cat food I suppose. We could also use them for furniture (sorry, I'm a bit unwell and consequently tired, but you can only nap so much when you're sick) fertilizer, not furniture, my mind escaped me for a moment. It would probably ruin the fishing industry. And it could potentially cause lots of other damages depending on how the fish show up, because it's clearly not in the lakes, rivers, oceans, etc. etc. because it wouldn't be "all" if that were the case, because not everyone has access to bodies of water. So how on earth would the fishes get here? And what kind of fish are they? Some kind of edible fish, other than that, we don't know. But that's all secondary because the point of this metaphor (metaphor? That's not a metaphor. It probably has a name and the name isn't metaphor. But I don't know what the name actually is.) is wishes, not fishes.
But what kind of wishes are they? All sorts I imagine and definitely not just the ones that start with "I wish", because I sincseriously doubt that there are enough "I wish" wishes in the world to provide everyone with king size servings of fish. What qualifies as a wish? Could you think to yourself "I could really go for a hamburger right now" and then have a Codfish pop out of thin air? Or what about "I need money for next month's rent." Is that going to cause fish to rain from the sky? What is a wish?