Well, there are two problems. One with the English language, and one in describing the word in the hints in an unambigous way.
So first: forms of "twój" are for singular 'you', aimed at one person. Forms of "wasz" are for plural 'you', aimed at more people.
But what's confusing is that 'singular' part in the hints for "wasza". It means that it describes a singular feminine noun. That it is your fish, just one, and not your fish, seven of them. Actually the specific noun used here adds to the confusion, having the same singular and plural in English...