Do some research before going out and buying just any starfish. Most that are readily available don't eat algae and often starve to death within a year. Some of them are carnivorous and others we don't even know what they eat. Generally speaking, brittle stars are a much safer bet. They tend to be omnivorous scavengers... but again, do your research because some of them get huge. This site is great for free information
http://liveaquaria.com/product/aquar....cfm?c=497+528
I'm a big fan of tuxedo urchins, tiger tail cucumbers and abalone.