"Fish is meat."
Translation:Ryba jest mięsem.
I can see they have now changed the text in the notes for this lesson to:
"When it comes to neuter nouns, you have to delete the final -o and replace it with -em (or -iem after some consonants)."
They also need to make that change for the masculine nouns (in this Instrumental case).
I also found this spelling rule which helps explain some of the times at least the the -em needs to become -iem:
"4. The letter e is usually separated from a preceding k or g by i, indicating a change before e of k to k’ and g to g’: jakie (jak’e), drogie (drog’e)."
Swan, Oscar (2008-10-12). Polish Verbs Essentials of Grammar, Second Edition (Verbs and Essentials of Grammar Series) (Kindle Locations 201-203). McGraw-Hill Education. Kindle Edition.