[cantSpellSober]

Does "The tank is too full, fill it with fewer water next time" sound correct to you?

"Fewer" typically refers to countables, "10 items or fewer."

"Less" is used for things that don't have a plural form or are not countable, "there's less water in the pool this morning."

[etothepii]

The opposite of few maybe many, but many is not the opposite of fewer as the parent asks.

Consider

There are fewer people on the West Coast.

There are many people on the East Coast.

"Many," is not a comparator.

[cantSpellSober]

Fair enough, "additional" is an opposite of "fewer." I'll concede "more" is also.

> The opposite of few maybe many

"Maybe" is an adverb. Perhaps you meant "may be?" :)

[mc32]

Morether. Like further but with more.

[teddyh]

Moreover.