"Your fish are fat."
Translation:Pisces tui sunt pingues.
Short answer: piscis is singular, pisces is plural.
If we change the English sentence to 'Your fish is fat.' -> Piscis tuus est pinguis
You can see which grammatical cases each can be used for at Wiktionary: https://en.wiktionary.org/wiki/piscis#Noun
Note that piscīs (a second accusative plural listed) will likely not be taught in this course (at least currently).