Agree about the shark. I'd want to be increasing your numbers of cories, as three isn't really enough to be a shoal and they'll be much happier in a group of 6=, as would the 'dwarf plec', if it's an otocinclus.
If it is indeed an otto like fluttermouth says then increase the numbers to around 6, this will really show the character of the otto and it will be much happier. If it is a Bristlenose then these guys can grow to around 5" so maybe be too big for your tank. I would go for 3 more Corys Get rid of the shark (Or get a bigger tank) 4 More Harlequins