the cut in the tail should heal on it’s own to help her not be stressed and picked on by the male you should get a few more females

other than this there isn’t alot I can advise you to do based on the information you have given the new fish that died real quick sounds like they were not floated long enough before letting them out of the bag

Common symptoms of stress include:

Fish stays near the surface gasping for breath, indicating that it has trouble getting enough oxygen (the concentration of dissolved oxygen is highest near the water’s surface). Possible causes include low oxygen concentration due to poor water circulation, toxins that have damaged its gills, high ammonia or nitrite levels, etc.
Fish won’t eat, or doesn’t eat as aggressively as in past.
Fish stays hidden continuously and won’t come out where it can be seen. Possible causes: aggressive fish, insufficient cover (e.g., plants, wood, etc.) to make fish feel “safe” while swimming about.
Fish has nicked fins, open wounds that don’t seem to heal. Possible cause: fish is target of aggression. Normally, minor nicks and cuts heal quickly. If they don’t, stress levels may be suppressing the fish’s immune system.
Fish has disease (parasites, fungus, etc.) Of course, the disease itself is a major problem. But in most cases, a healthy fish’s immune system keeps it from getting sick in the first place. Thus, getting sick is a sign that the fish is in a stressed state (or had been until recently).

Tips on keeping your fish from being stresses:
-Keep in mind that if your fish is sick, it’s stressed. And stressed fish are more likely to get sick.
-Know that a fish whose wounds don’t heal is having problems. Make sure it doesn’t have parasites. Watch to see if it’s the victim of a bully in the tank. Make sure that there are plenty of hiding places.
-Make sure you’re giving it a variety of foods and that it isn’t having to compete to eat if your fish has lost its appetite.
-Give your fish space. Make sure that the tank is not too crowded. Like people, fish need their space.
-Be sure that the species in your tank are compatible.
-Keep the tank at optimum temperatures and the equipment in good condition. Fish need oxygen; some fish are more tolerant of low-oxygen water than others. A fish that stays near the top of the tank and gasps for breath is telling you that it’s having trouble breathing. Check the ammonia and nitrite levels, the filters, and the fish’s gills.
-Never add untreated tap water to the tank. Chlorine and chloramine are toxic to fish.
-Keep the tank and filters clean. Poor water quality, especially levels of ammonia and nitrites, and the pH can really put a fish in a bind.
-Find out what your fish like. Some prefer hard water over soft, some prefer acid over alkaline, and some do better with a little bit of salt in the water. Are they solitary or school fish?
-Make sure the tank doesn’t get too small for its occupants. Crowding can lead to increased aggression.
-Use extreme caution when adding medication to the tank water. Any change you make to the chemistry is going to affect everything living in it.