Water conditioner will not help with what I referred to as bioload. Do a read on the nitrogen cycle. This site makes it pretty simple. http://wwwDOTtropicalfishcentreDOTco.uk/Cycle.htm (I hate this forum. It always edits out other people's URLs trying to keep people from looking at other sites.)...
Water may appear clean, but it may be quite toxic for fish.
Ammonia, nitrites, and nitrates are part of the nitrogen cycle, all referred to as bioload. Fish waste starts as ammonia, bacteria converts it to nitrites, and then bacteria converts nitrites to nitrates. Nitrates are generally removed...
Although you never say it in your, I assume you are looking for suggestions as you don't want to lose this fish.
We really have to take your word on what you're telling us as you are giving incomplete information. I'm not trying to accuse you of hiding anything on purpose, but can you tell us...
Her tail will grow back. You will want to treat her with antifungal medication to help the healing process. Be aware that some medications contain dyes and will stain silicone and ornaments in your tank.
She was probably not attacking them. She was trying to eat them. That's what they do. They eat their fry. The reason you don't see a lot of dead fry in breeding boxes and nets is because they're normally eaten pretty quickly. Unless you get to witness the birth or shortly after, you won't see...
Loachman is right. Personality-wise, they will do fine. Guppies and Mollies can interbreed, but it's not that common, and if they are balloon Mollies, I would suspect it is even less likely. That shape is another obstacle to overcome.
Place your thermometer on the far end from your heater. That way you're getting a true tank reading. If you want your heater to heat the tank evenly, placing it under the filter outlet is a good idea.
1. What kind of fish is piper?
2. What are your water parameters?
3. Any other signs of illness?
Try target feeding. A syringe and bloodworms might help.
Good questions. Are there any other fish you have in the tank that do not die? You really left little information to discern anything. How large is the tank? Is it cycled? What are your water parameters for ammonia, nitrite, and nitrate? How is the temperature? It may be poor conditions the fish...
Congratulations on the fry. The initial fry were probably born dead, aborted. This is common when using breeding nets. It's not always the case but the stress can cause fish to abort the fry.
Swordtails and Platies.
Mollies, Guppies, and Endlers.
Livebearers can store sperm and give birth up to an average of six times without a male present. That may be what you're seeing.