"Tamta ryba jest mała."

Translation: That fish is small.

[PuertoRico_7213]

Can you also say małą

[Vengir]

That's very unncommon and sounds awkward to some, better not to use it. The adjective should agree with noun in its grammatical case..

[PuertoRico_7213]

Thank you

[winandfx]

Is it instr. case (małą)?

[immery]

yes małą is instrumental singular feminine.

https://pl.wiktionary.org/wiki/mały#pl

[ionradoi1]

tak

[immery]

You may say it, you might hear Poles say it, it is not correct, unless before a noun.

[PuertoRico_7213]

Thank you :)

[Kaktusik3]

With małą the sentence:
Tamta ryba jest małą rybą

[BjrnWikan]

How come it's 'that' and not 'this fish'?

[Jellei]

Well, first you can check what immery usually writes to questions about this/that: here

So we usually accept both 'this' and 'that' for 'ta' (at least we try), but if it's 'tamta' in Polish, it really needs to be 'that' in English.

[LucreziaBorinato]

Do adjectives match with their nouns in their grammatcal case even after verb "być"?

[Okcydent]

Yes. Adjectives match nouns. If there is no noun after być the adjective is in nominative.

[ArmenO7]

Why no instrumental case in such sentences?

[Jellei]

You use Instrumental in such sentences for noun phrases, but not if you only have an adjective. Maybe it will be easier to show with a masculine noun:

"Lew jest duży" (Nominative)

"Lew jest dużym zwierzęciem" (Instrumental)