Oto stands for Otocinclus, with the most common being Otocinclus affinis, do a web search on google and you'll get plenty of pictures. They're a type of catfish a hence I'm pretty sure they're strictly freshwater. They make a good clean up crew but need to be kept in groups of at least 4 so you need to make sure that you're tank can accomodate this number of extra fish. Personally I feel that if you're going for a FW tank and want a clean up crew you're better off getting an Ancistrus (Bristlenose Pleco).