Here’s one of those rules to live by that many hobbyists tend to break on a regular basis. Clownfish are hugely popular in the aquarium trade, and they’ve pretty much always been, even before ‘Finding Nemo’. Many aquarists buy these fish in pairs, for some reason they are so heavily marketed as such, but a lot also buy several more. It’s not uncommon for a hobbyist to have 4-6 clownfish in a single aquarium, and we regularly see people mix species. Unfortunately, this never ends well. Clownfish are very friendly when they are young. They do well with every fish in the tank, even other clownfish. But as they get older, their natural instincts to assert dominance usually lead to significant fighting, often leading to a single juvenile turning into a large, dominant female that terrorizes everything else in the tank. This is particularly common among maroon clowns, but also prevalent in the more docile species. Long story short, it is recommended that you keep clownfish singly, or as a pair from the same species. Do not mix clown species, even in large tanks.