Most of my students think this question is wrong and want to change "its" to "their" since they want to match this pronoun to the plural "the young fish" However, "its" is actually referring to "each" and it works because "each" and "its" are singular. In doesn't have anything to do with the error ( or lack or error)
"in solid food every day" is not modifying anything, it is part of "its own weight" it is a preposition within a adverb clause. I think. I don't know the term for the "its own weight", but I know that the "in" is within that clause, not excluded from it. At least that is how I learned it in latin.