5/8"OD Y connector would better than 1/2"OD, since it will be less restrictive...
You can insert 1/2"ID premium quality tubing (ClearFlex 60, Tygon R3603 etc) over larger 5/8"OD connector fairly easily... Just heat up the ends of tubing a bit with a hair dryer @ low setting, or just insert the ends tubing in to boiled hot water...
Bruce
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