In Ephesians 5:9, the textual variation involves whether we should read 'for the fruit of the Spirit is in all goodness, righteousness and truth' (as in the KJV and NKJV) or 'for the fruit of the light is in all goodness, righteousness and truth' (as in most other modern Bibles). The first reading 'Spirit' is found in our oldest witness at this point (P46, c. A.D. 200) as well as the majority of Greek manuscripts. However, the second reading is found in about twenty manuscripts, including the early P49 (3rd. cent.). But crucially, this second reading is found in the majority of versions, including the Latin, Syriac and Coptic versions from the east, west and south. Only one later version (the Syriac Harklean version) aligns with the first reading. It seems less likely that the transmission of the NT text into all the different versions of antiquity would independently hit upon the same wrong reading than that the transmission of the majority of Greek manuscripts was corrupted at this point.
The other big point in favour of the second reading is that it fits with the context. All through the first part of Ephesians 5, Paul is writing about how Christians should be 'light in the Lord' (5:8), that we should not 'have fellowship with the unfruitful works of darkness' (5:11), that 'all things are exposed by the light' (5:13), and that 'Christ will give you light' (5:14). The 'fruit of the light' seems more in keeping with what Paul is writing about than the 'fruit of the Spirit', which instead seems to be a harmonization to Galatians 5:22.
Therefore, on grounds of propinquity, internal logic, and transcriptional probability (the possibility of harmonization to Gal. 5:22 here), we should strongly prefer the reading found in the more geographically diverse witnesses, 'the fruit of the light'.