Mark,
I think i might need to capture some shots of my fish. In short C.siamensis does not even exist. There is no such species, but it is an umbrella term for a few closely related individuals. They all eat algae to varying degrees,but the one you want is C.langei, the other commonly available fish under C.siamensis is C.atrilimes, but this fish has a taste for higher plants; moss in particular. They wont be sold as their proper species though so you can only learn to differentiate. The easiest difference to my mind is C. atrilimes has a shorter "snout".
All algae eating fish get lazy when there is easier food to be had, but these are some of the best. C.langei especially for BBA.