Hello, I would like to begin by saying that I am a young male and keeping an aggressive fish appeals to me at this age. I have a 5 gallon tank that I would love to put something nasty in, a species that ins't particularly friendly and doesn't mind being alone in a small space. Please don't recommend a betta, I have had those before and I don't really want any more. I was considering a small species of crayfish or freshwater crabs, but I thought I would ask if there were any "fancier" options that anyone might now of.