"Η μείωση της ποιότητας."

[Howard]

I prefer "The decrease in quality" (accepted).

[jaye16]

Yes, "...in quality" is also a correct translation. Thank you.

[Theofa]

"Τhe reduce of the quality" Don't accept! Why???

[jaye16]

You need the noun "reduction" here. "Reduce" is the verb.

[Theofa]

Thank you!! You need the noun.