Yes, killifish are fin nippers. Killifish are usually peaceful fish that can become aggressive when they are kept with the wrong tank mates or in an inappropriately sized tank. Male killifish are best kept separately and tend to be more aggressive than their female counterparts.
Providing your fish with plenty of swimming space will lessen the likelihood of your killifish nipping their tank mates’ fins. One pair of killifish needs a 20+ gallon tank.