Learn Arabic (Modern Standard): هذا السمك ألذ طعما من الدجاج.

This is the Accusative of Specification (التمييز). It tells us exactly in what way the fish is better. After a comparative adjective like ألذ (tastier), you can add an indefinite noun in the accusative case to clarify the comparison. Literally, it translates to 'more delicious in taste.'

Is طعما strictly necessary here to mean 'tastier'?

No, it is optional. You could just say هذا السمك ألذ من الدجاج (This fish is tastier than the chicken). Adding طعما is a common stylistic choice in Arabic that makes the language sound richer, specifying that it is more delicious specifically 'in taste'.

The comparative pattern is usually 'af'al' (like أكبر). Why does ألذ look shorter and have a shadda?

The adjective for delicious is لذيذ, which comes from a root where the second and third letters are the same (ل-ذ-ذ). When putting this into the comparative pattern, the two identical letters merge into one with a shadda, making it ألذ (aladhdh).

If I leave out the 'al-' (ال) on السمك, does the meaning change?

Yes, it completely changes the sentence structure! هذا السمك (with 'al-') means 'This fish'. If you say هذا سمك (without 'al-'), it forms a complete sentence meaning 'This is a fish,' and the rest of the sentence wouldn't make grammatical sense here.