Adjectives used as nouns
An adjective used as a noun is capitalized and still follows adjective declension: der Kranke, ein Kranker, die Kranken. After etwas and nichts, a nominalized adjective usually takes -es: etwas Neues.
Understand the idea
An adjective can act as a noun, especially for a person or an abstract quality. It is capitalised but still takes adjective endings. Its ending therefore depends on the article and case rather than behaving like an ordinary fixed noun suffix.
Build it step by step
Compare a definite article, an indefinite article and no article. After etwas or nichts, a neuter nominalised adjective commonly has -es, as in etwas Neues. Words referring to people can change endings across gender and case: der Kranke differs from ein Kranker. The surrounding phrase tells you which pattern applies.
See it in a sentence
Ich möchte etwas Neues lernen.
Er hat nichts Wichtiges gesagt.
A useful memory aid
Keep the article and capitalised adjective together when studying the form. Its noun-like role does not erase its adjective-like declension.
A common trap
Do not freeze a dictionary form such as Kranker in every case. Also remember the capital letter when an adjective is actually being used as a noun.