Fish の複数形は、fishes じゃないの？

"Look daddy!!! Our fish had 6 baby fishes!!!" my son said to me.
I went over to the corner and peered into the soccer ball-sized fish tank and indeed there were 6 little ones. However, when we talk about fish the plural form (複数形) is fish. One fish, two fish, three fish, etc... Let's use it in a situation.

Your friend, Shuichi tells you, "I went fishing last weekend."
and you ask: "How many fish did you catch?"
Shuichi: "I caught 8 fish."

Sheep and deer follow the same rule. The singular form (単数形) and the plural from are the same.
One deer, two deer. One sheep, two sheep, three sheep, zzz...