Fishes is not an acceptable plural as used here. It is not used in everyday communication, only scientifically, as the plural for many different species of fish.
As a matter of interest, this Duolingo exercise displays the vagaries of translation. The Spanish sentence actually specifies more than one fish. It's not "pez" (singular), it's "peces" (plural). But with a twist of irony, the reverse English translation is actually ambiguous. If you were to reverse-translate “The fish left home” to Spanish, you could mean (one) fish left home or (two) fish left home. So of course, translating English-Spanish, with "fish" (singular and plural), you’d have no way of knowing the number of fish without further context.
English has many "irregular" plurals which don't add a simple "s" or "es" to the singular to form the plural. Each has to be learned and memorised.
You’ll find some examples here: https://grammar.yourdictionary.com/grammar-rules-and-tips/irregular-plurals.html