They can be, so it's something to watch out for. They would do better in a tank where their tankmates are at least as aggressive as they are, passive fish will get bullied.
As an example, I established a 90L tank and moved 6 x serpaes in to start with. I later added 2 x German Blue Ram juveniles, and the tetras went straight after the smaller female and attacked it savagely. I quickly grabbed the net and managed to catch 1 of the serpaes, which I put in a transport bag. This unsettled the remaining tetras and they then acted "unsure" as new fish do when you add them to a tank. After a couple of hours allowing the tank to calm down I reintroduced the tetra and the GBRs didn't get attacked again. 2 days later, the GBRs laid eggs (!!!) and the female in particular was chasing after any tetras that came within range. They haven't laid again, unfortunately, but the female will assert herself and chase tetras when she feels like it. The tetras were territorial at first because at the time, they felt like they owned the tank having been the only inhabitants, and the GBRs were on their turf.
The tetras also went through a phase of chasing some bronze corys I added, but now the red-tailed black shark I added at the same time as the corys has taken a liking to chasing the tetras (must be a good judge of character!!). The hierarchy in the tank is quite interesting, the GBRs are clearly top rung which is the way I like it. They are my prize possession - whoever attacks my GBRs is getting flushed down the toilet
(just kidding.. they would get rehomed though!).Massive tangent.. but the heirarchy in my tank is now:
1) GBRs
2) BN catfish (minds his own business but when he wants food the only fish he won't snap at are the GBRs)
3) RTBS
4) Serpae Tetras
5) Siamese Algae Eaters
6) Bronze Corys (poor corys, they are so dumb! haha gotta love em tho)
