This came up in another thread, and i know it's something that i didn't learn about until i'd done a lot of reading in the rec.aquaria newsgroups, so i thought i'd post some info here.
To keep algae problems under control in an aquarium, it is essential to have some fish that eat algae. Some do a better job of this than others, and some are especially sought after because they eat algae that no others will eat. One example of this is the Siamese Algae Eater (SAE), Crossocheilus siamensis.
There are several fish that are very similar in appearance to the SAE, and often sold under that same name. Unfortunately, these fish not only aren't very good algae eaters, but they can become bullies towards your other fish.
The Krib, an excellent aquarium information site, has this page that explains in great detail these different fish, and how to tell them apart. Some are easier than others. The hardest is probably this one,
Notice that the black bar doesn't continue into the transparent tail fin. That's the only obvious (to us ignoramouses) difference between this fish and a true SAE.
Here are a couple pictures i took of my two SAEs, so you can see the difference:
Note the distinguishing characteristics: the somewhat jagged black bar, the dark-outlined brown scales above the bar, and the extension of the bar into the tail. There are others listed on the krib page, so be sure to check it out.
Good luck with your algae, and by all means, avoid the so-called "Chinese Algae Eaters"!!!