"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).
I suspect you want to use vestri, the possessive adjective. Normally the course will accept singular or plural (unless it say 'you all'), just report it if there it doesn't accept one or the other.
Vobis is the dative or ablative 2nd person plural. The dative could work as is (dative of possession construction) but the meaning would be closer to "You have fat fish." if I am not mistaken.
tuus, tua, tuum -> 'your (singular, one person)
tui is the nominative plural here.
vester, vestra, vestrum -> 'your (plural, multiple people)'
vester is the masculine nominative or vocative form of the adjective. You would have to use vestri here since that is the masculine nominative plural form.
Either tui or vestri should work here as it does not specify if the 'your' is plural or singular.