What I use but dont recommend this:
Beduan Brass Hose Barb Reducer, 5/8" to 1/2" Barb Hose ID, Reducing Barb Brabed Fitting Splicer Mender Union Air Water Fuel https://www.amazon.com/dp/B07PNK3SZY/ref=cm_sw_r_cp_api_fabc_lqHPFbK1QS5H1?_encoding=UTF8&psc=1
What I would get now:
Jardin 12mm to 16mm Hose Adapter Converter for Aquarium https://www.amazon.com/dp/B0089L5G42/ref=cm_sw_r_cp_api_fabc_cCHPFb2A8R02R?_encoding=UTF8&psc=1