Hello all, so I am having a problem with my Black Skirt tetras. I have 4 of them and two of them have become very aggressive, they bully the other two docile ones, and have seem to set up their own boundaries in the aquarium. I have had the 4 of them together for almost 2 years without any problem, just now they started to act like this.Will adding 2 more black skirt tetras fix this? Other tank mates are 6 red minor tetras. I have a 20 gallon tank with and heater and eheim 2213 filter system.