Me and my roommate have a (marketed as chili, but likely dwarf or strawberry from the markings) rasbora who we think is a female in a 5-gal planted nano tank. Her tank-mates are assorted small snails, a horned nerite snail, and four small neocardina, we plan to get a few more after winter. She was originally in a school of about 8 but we had problems with losing a lot of fish, possibly from bad breeding or not cycling the tank enough as we are beginners to all this (tank is abt 6 months old now). Ended up with two who were the biggest of the pack surviving, and lost the other a few months ago (did not find the body, assuming our Neocardina ate it). My roommate who was keeping the fish at the time did report that the two who made it were sort of bullies, but they’re a timid species so didn’t put much stock into it. The remaining fish has been generally in good health and very nice coloring as of late, if a little more active and flighty than expected.
The problem is that the fish will often follow around our two red cherry shrimp as if attempting to school with them, which has been comical up until today when I spotted her attacking our oldest Sakura-grade shrimp, they’ve both been tank mates since the beginning of the tank and are the two surviving original members, he’s bigger and far more colorful than her. She nipped and chased him around the tank for a few seconds before he started writhing and fell to the ground upside down, in a small spot where she couldn’t get to him. It took him almost a full minute to right himself and one of his legs seemed motionless. He slipped into a hide after and seems ok now but I’m worried if this behavior may continue with other red shrimp or even if we choose to get her some new rasbora friends (we feel bad she’s all alone, but at this point I think the reason she’s alone is that she’s mean and stresses other fish). Has anyone seen this behavior before or has any advice to mitigate it? We just want our tank to make it without lore casualties and any comment is very appreciated.