I combined a twin-fin beta with a orange stickfish as in the magic fish guide and thought I'd get a specked leffish. Instead I get a spined leaffish!

What am I doing wrong?

That is one of the combinations for the Speckled Leaffish for most versions of the game. It's easy to accidentally breed the wrong fish, since sometimes you'll aim for one and grab a different one. Try the combo again, carefully, and see if you get the same result.

Which version of Fish Tycoon are you playing (Windows/Mac, DS, free online (Flash), Palm, Pocket PC, or iPhone/iPod)?

Windows version.
I'm fairly sure I picked the right fish as I've ended up with several spined leaffish as I made several orange stickfish pregnant, but I will try again.

Put your spined leaffish in the isolation chamber and click the Species button. You'll see the "family tree" showing the parents.

It seems you were right. I'd picked up a greenfin beta by mistake.
I will try again!