The English language is weird...
Some words that are spelled the same can mean completely different things, and others that sound exactly the same can be spelled differently to alter their meanings. There are "rules" for referencing multiples of something, but they seem to be more like "strong suggestions" rather than strict rules.
For instance, if you have more than one goose, you have geese, but if you have more than one moose, you don't have meese, you still just have multiple moose. Some people believe the plural of octopus is "octopi" or "octopodes", but the only grammatically correct way to refer to them is "octopuses".
How, then, is one to reference plural plurals?
"Fishes" may sound like a really weird word, but it has a purpose that is actually much easier to understand than one might think.
If you have more than one of a single type of fish, you still only have fish. However, if you have more than one of multiple kinds of fish, they can be collectively referred to as fishes. The same rule applies to "people". You are a person, a group is people, but multiple groups are collectively known as "peoples".
Sounds weird, right? Regardless, this is the English way...