She is happy alone. If she was ever put with other serpae tetra she gets bullied or she is the bully. She was living happily with the guppies for 7 months.
I'm afraid this is not true. This species lives in large groups, and this need is imprinted into the genetic blueprint of the species. The fish "expects" to be in a group, and if denied this necessity it will be stressed. There is now scientific proof for this.
As for being bullied, or bulling others, that is what happens when the fish are not kept in a sufficient-sized group. And in sufficient space. This species needs 12+ fish, in basically a 30 gallon tank (one with the base dimensions of 30 by 12 inches). When these needs are met, there is less chance of bullies. However, individual fish can be exceptions to the norm, it happens in all species.
No one can tell if a fish is "happy" because we cannot talk to the fish to know. We can assume that by understanding its needs as I've outlined above and providing these, it
should be healthy. But denying these necessities is not being kind to the fish. Read Dr. Loiselle's comment in my signature block.
And now that this specific has been raised--it is very possible that this is the cause of the issue here. This fish is severely stressed. Fish have little option under such conditions, they either become aggressive, or the opposite. "Swim bladder" is rarely the issue, as this particular symptom of the fish being unable to keep itself buoyant can be brought on by several things such as stress, internal protozoan, certain foods, genetics, inappropriate or fluctuating water parameters, and water conditions.
