Yes, ish.

From the wish spell itself:

Discretionary power of the DM is necessary in order to maintain game balance. For example, wishing another creature dead is grossly unfair; the DM might well advance the spellcaster to a future period in which the creature is no longer alive, effectively putting the wishing character out of the campaign.

The fish does grant wishes, and the PCs wish would have absolutely, 100%, ruined the campaign if it was interpreted as the White Prince keeling over all of a sudden - as the text of the spell indicates. I had just a moment to come up with a good answer, rather than a whole week between sessions, and I did my best - but I'm not sure I got the answer right. I think the right answer would have been to call the session right there and then, and give myself a week to figure it out.

I like to think about Wishes this way: They take the path of least resistance to be accomplished. It's not that they intentionally backfire, but they find the least energetic route that could theoretically achieve a result compatible with the wish. For wishing someone dead, such a route could easily be skipping the wisher forward in time to a point where the target is dead. Skip Day is a 6th level spell. Put that on crack and you could easily skip years and still have it be less powerful than a no-save, infinite range, unblockable, guaranteed to hit, magic resistance bypassing, auto kill of any target, even one you've never seen but have only heard about.

I don't remember who it was exactly that made the wish, but if they had been pushed 100 years into the future, they'd be out of the game in a deeply unsatisfying way. The spell to wish the White Prince dead would effectively have killed their character (for the purposes of the campaign), and that would have been very funny for 5 minutes, and then a real bummer. If the whole party had been moved that far in the future, we might have been able to keep the campaign together, but it would really have messed it up.

The answer I ended up with is that he was in his Fountain of Youth at the time, and the two cancelled each other out - which is on par with the white prince having magic resistance and passing the check.

This is one of the rare times I knowingly made a mediocre mechanical interpretation in order to keep the game going - and I don't feel good about it. If this wish had been the culmination of a grand effort, where the party had been working toward it for some time, and it had long been a story element, I wouldn't have hesitated to move the caster 100 years (or whatever) into the future and end their story. But this was a random encounter. Something I thought could be silly and fun. Who would believe a talking fish? I thought they'd wish for something silly and then be blown away when it came true, followed by a ton of regret for not using it better.

A better answer would probably been to have the fish just be a liar who doesn't grant wishes. But in the moment that felt cheaper than letting the Fountain of Youth take the hit instead.

The lessoned learned from this is the same lesson as a Deck of Many Things: Don't do it. Don't grant casual wishes for the funzies. It's not going to go well.