Don't feel like a moron...it's the manufacturers who are to blame. The 1/2" is apparently nominal and there are minor differences between different brands...but just big enough to make some fittings incompatible. (Probably intentionally so.) It's easiest to stick with one brand for all your 1/2" stuff, but you can probably take care of your current dilemma by using a double-ended barb fitting and hose clamps to crank down around the tubing.

kudzu9 -

Thanks for the response.

You're certainly not stupid for being confused. The crux of the problem is the metric v. English measurement systems. So-called "1/2 inch hose" can be either 16 mm (0.620" OD) or 18 mm (0.720" OD). Both are referred to as 1/2" hose, so you must be very careful to discriminate.

Choose one or the other and stick with it, but that should explain why 1/2" ain't always the same!