Why is this genitive
Also salsi is only singular
Nothing here is genitive. illi, pisces, and salsi are all nominative plural. salsus is the masculine form of the adjective and is 2nd declension which means salsi can be a plural form.
I wrote 'pisces illi' and it's wrong! The demonstrative in latin can be after the substantive!
I am doing Latin drunk at the moment (sorry, ebrius sum), but I'm fairly sure 'pisces illi salsi sunt' should have been right, not wrong.