Good morning, Moniki.
(1) What an interesting question.
(2) Well, first, I think (think) that most Americans usually use "fish"
for the plural.
(a) About the only people who use "fishes" for the plural may be
scientists who are studying the various kinds in the ocean.
(3) As you know the word "a" comes from the word "an," which
(a) I think speakers would use "one" only if they wanted to emphasize
Tom: How many pets do you have?
Mona: Well, I have three birds, two cats, and a dog.
Tom: Excuse me. I didn't hear you. How many dogs?
(4) Therefore, in my opinion only, I think that if you and a friend
are having lunch and talking:
Martha: I have three birds, two cats, and a snake. What about you?
You: Oh, I have two fish and a dog.
***** Have a nice day!
Student or Learner