I want a small aggressive bottom dwelling,nocturnal fish specie that grows to 6 inches or less.

As a rule bottom dwellers are not normally aggressive, but many are more then capable of holding their own in an aggressive aquarium. Some good choices would be Synodontis petricola,and Synodontis schoutedeni also there many species in the family Doradidae that would fit you specification.

But if you want a fish that is out and out hostile I would go with Skunk Loach (Yasuhikotakia morleti) or Blue Botia (Yasuhikotakia modesta).

Define aggressive. Do you want a territorial fish, a predatory fish?

I want a predatory aggressive fish,but i would also like it to be a little aggressive.

Some good small predatory bottom dwellers would be Leiocassis siamensis (Bumblebee catfish), small members of the family Pimelodidae (Pictus cats), and Mystus species.

But I think what would work for you is Tetraodon miurus (ugly, mean, & predatory)

I would say the Arrowhead Puffer. I had a nice 4-5in that would literally shred apart and completely eat goldfish and any other fish its own size. Yup!!! I'd go with the Arrowhead Puffer (aka-pig nosed puffer).