Just to echo AMS's comment above with a warning: the name "Hypostomus plecostomus" is often used in aquarium books and fish stores for the common plec --- but this isn't the correct species name. The common plec in the hobby can be anything really but is usually something like Pterygoplichthys multiradiatus or Pterygoplichthys pardalis.
Other brackish water algae eaters include mollies, Florida flagfish, and violet gobies. In fact most non-predatory brackish water livebearers and killifish eat a lot of algae, so buying any of them will be helpful. Mollies in particular will do a very good job of eating filamentous algae. Stiphodon gobies are prodigous algae eaters, but they aren't easy to keep.
It's worth mentioning that, contrary to popular belief, you DON"T NEED scavangers. Adding any fish, even a scavenger, reduces water quality by adding biological loading. It is ALWAYS better to clean a tank by siphoning out waste on sight. If you don't want to do a water change while doing this, simply pour the water back into the tank through an aquarium net, and you'll remove solid waste very effectively. I do this in a tank containing a wood eating catfish who produces masses of wood chippings every day, but I can't afford to change the water every time because the soft water used is rainwater, and sometimes the rainwater butt is empty!
Cheers, Neale
