"This is a tasty fish."
Translation:To jest smaczna ryba.
"To" isn't modifying "ryba" in this sentence! It doesn't say "this fish" (though that could be implied). You're not picking this fish from among other fish, you're picking this thing from among other things (and saying that it's a tasty fish).
If you were to say "this thing" in Polish, it would be "ta rzecz," not because "ryba" is feminine, but because "rzecz" is. But as in English, you're allowed to use what linguists call a "zero mark" or "∅" here (i.e., leave out a normally expected noun).
Imagine for the sake of illustration that "∅" were a neuter Polish word meaning "thing." To agree with the gender of ∅, you'd say "to ∅ jest smaćna ryba."
Think of it the same way. The zero noun here happens to be formally neuter, so no matter the gender of the other thing the "jest" attaches to, you'll still say "to jest."
Actually it depends on the noun's gender. "to" can be either a dummy pronoun (like in "[This/That/It] is a tasty fish") and then it is used with absolutely any noun. But it can also be a neuter singular determiner (like in "this child" = "to dziecko").
So for neuter nouns, like "To smaczne zwierzę", it can be indeed ambiguous and it can means both "this tasty animal" and "this is a tasty animal".
For any other noun, using "to" definitely means "this/that/it is...", and you need to use another form as a determiner. For example "this tasty fish" is "ta smaczna ryba", as "ryba" is a feminine noun.