Yes, killifish are aggressive. Killifish are usually passive and peaceful pets that can become territorial and aggressive depending on their environment.
Male killifish most commonly demonstrate aggressive behavior and tend to behave aggressively when with other male killifish. Killies may also eat tank mates, eggs, and fry that can fit into their mouths.
Make sure to separate your killifish if they become aggressive. You can keep them in community tanks, but they will act out on their smaller tank mates if you do not keep them in the right conditions. A 20-gallon tank or larger is good for a pair of killifish.
