Ok well , since you went out of your way to contradict me. Lets take a look at what he told us and how limited the information was when I said check for ammonia.
Why not check for ammonia I simply suggested it.

At any sign of fish stress (or perceived fish stress from a sleeping oscar) It is never a bad idea to do an ammonia test.
Now knowing what we do I would say goldfish an Oscar and a pacu (I realize this is temporary) could cause quite the bio load. Another reason to check for ammonia. You get where i am going with this ? The leading cause of fish death is from over feeding which causes ammonia and kills fish. When you think your fish are acting odd or better yet once a week run an ammonia nitrite nitrate test.
PS the whole "well somebody else said it" comment was a page out of your book so hang on to your gold stars