"The fish went out for breakfast."
Translation:El pez salió a desayunar.
You used "desayuno" (noun), instead of "desayunar" (verb infinitive). I think if you do this, it would need to be "al desayuno" or "un desayuno". It's hard to spot the difference because in English the noun and verb use the same word. A way that you can tell, is to replace "desayuno" (noun) with another noun. Let's use "restaurante". "We went out to restaurant" doesn't make sense. It needs to be either "a restaurant", "the restaurant", or you need a verb (we went out to eat at a restaurant). I hope this makes sense to someone other than myself.
Make total sense. Still, I think the main problem is that we treat "breakfast" in English as if its a location, and Spanish does not seem to like that, so one must say "go out to eat breakfast" in Spanish, hence the verb instead of the noun. The only exception to this I've found is with "el trabajo" which can be a location, but its usage is certainly not universal--my memory is that Duo prefers "trabajar".
Probably because "pescados" are dead fish that you eat. They would not tend to go out to eat as they are already dead. "Peces" are the fish that are swimming around in a fish tank or pond. Kind of like pets. Or are swimming in the sea before they are caught for consumption. That is just a guess on my part by the way.
Sentences using «[pez / peces / pescado / pescados]» are all in the list of Duo's Correct Solutions for this exercise.
The sentence is incorrect because of «a desayuno» as Tyka has pointed out.
The accepted options are:
verb «[a / para] desayunar»
noun «[al / para el] desayuno»
You have typed a sentence into the forum that has a match in the list of Duo's Correct Solutions for this exercise, and without seeing your screen I can't offer any reason why Duo rejected it.
If possible, please capture a screenshot to share with the user forum so that we can try to see what went wrong.
[GUIDE] How To create, upload, share a screenshot (using postimages.org)
For now, at least be assured that your translation here in the forum is good.
I think a live fish is required, so pez, not pescado. However the main problem is that fue would merely mean to go somewhere, losing the full sense of "going out", which is different. You could go the breakfast if you were staying in a hotel, and that would be fine. But if you instead breakfasted at a café down the road from the hotel, you would, of course, use salir. So, salió is best for "went out", not fue, yes?