Learn German: Die Korallen im Riff sind am schönsten, aber sie brauchen diese Härte. - The corals in the reef are the most beautiful, but they need this hardness.

It comes from the adjective hart (hard). To form the abstract noun, you add the suffix -e and apply an umlaut to the stem vowel, changing the a to an ä. Nouns created this way are always feminine, which is why it takes the feminine demonstrative article diese.

Why does the sentence use am schönsten instead of just schönsten or die schönsten?

When a superlative adjective stands alone after a linking verb like sind (acting as a predicate adjective), you must use am with the -sten ending. You would use die schönsten if the noun directly followed it, as in Sie sind die schönsten Korallen (They are the most beautiful corals).

Why is it im Riff and not ins Riff?

The preposition in is a two-way preposition. Because the corals are already located in the reef and not moving toward a new destination, it answers the question 'where?' (Wo?) and takes the dative case. Im is the required contraction of in and the dative neuter article dem.

Could the phrase sie brauchen refer to the reef needing hardness?