"He eats tons of fish."
Translation:Él come toneladas de pescado.
The reason is simply because pescado by itself is a noun. De pescado is an adjectival phrase, but we don't make a whole phrase agree in number with the noun it is modifying. Other examples:
Some bottles of wine. Unas botellas de vino.
Many slices of cheese. Muchas partes de queso.
Yes vino and queso are mass or uncountable nouns as is pescado. The previous explanation could probably be enhanced by saying that when talking about the quantity of an uncountable noun, the only word that would be put into the plural is the unit of measure you are using to count. With countable nouns, the countable noun is probably already in the plural once you introduce the unit of measure. Consider the following
A box of wine, two boxes of wine but
A box of cookies, two boxes of cookies
A ton of fish, two tons of fish
A ton of books, two tons of books.
This is a property of uncountable nouns and will apply to any language that has them.
I looked up "comer" and "comerse" in Wordreference. "Comerse" means to eat up or use up. Or figuratively it can be used for things like blowing through money. So, I think it would be like this: if I eat the last half of the 1/2 gallon carton of ice cream and there's none left for anyone else, that's "comerse." If I eat a 1/2 cup serving of the ice cream, that's "comer." I suppose if someone gobbled down a ton of fish leaving nothing, that would be comerse. If someone likes fish and eats it three or four times a week, but always in moderation, that would be comer--but might seem like eating a lot of fish to the majority of us.
Anyone else have thoughts on this? Any native speakers out there?