[I am not a teacher.]
The word -fish can have either of the plural forms as below:
#1 fish eg. 10 fish
#2 fishes eg. 10 fishes
fish - Definition from the Merriam-Webster Online Dictionary
So is jellyfish
#3 10 jellyfish, a lot of jellyfish
#4 10 jellyfishes, a lot of jellyfishes
jellyfish - definition of jellyfish by the Free Online Dictionary, Thesaurus and Encyclopedia.
Please click the URL links and read the useages of the words. The dictionaries indicate the plural forms clearly.
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