You need to squishy up your signature a bit, it takes up more space than the question
If you can post a clear picture of the fish with the red bit by her tail it might help. Also a short 20 second video of her showing her breathing and sitting under the surface.
Stringy white poop can be caused by the following:
Internal protozoan infections causes the fish to go off its food and lose weight (get skinny) over a week or so and they do a stringy white poop.
This can sometimes be treated with Metronidazole, but I don't think this is the issue.
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Internal bacterial infections cause fish to stop eating, bloat up suddenly overnight, do a stringy white poop, sit under the surface or near a filter outlet, and breath heavily. These fish usually die within 24-48 hours of showing these symptoms. There is not normally any cure for fish with internal bacterial infections and fish normally die from organ failure.
This might be the issue if she has stopped eating.
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Intestinal worms (tapeworm, thread/ round worms) will cause fish to initially lose weight over time (a month or more) but this is hard to tell in female livebearers due to the babies they might be carrying. Fish will eat normally but do stringy white poop sometimes or all the time. In severe worm infestations, the worms can build up in such numbers the fish actually becomes fat and looks pregnant, however it is full of parasitic worms feeding off the fish's blood.
Intestinal worms can be treated with Praziquantel (for tapeworm and gill flukes), and Levamisole for some types for thread/ round worms. You treat all the fish in all your tanks at the same time. You dose them once a week for 3 weeks to kill the adult worms and any young worms that hatch from eggs in the fish. Both of these medications are safe for all fishes and do not harm the beneficial filter bacteria.
Use one medication at a time (eg: Praziquantel) and then do a 75% water change 24 hours later and again 24 hours after that. Then treat with Levamisole and do a 75% water change 24 and 48 hours after that. A couple of days later (a week after you initially treated the tank with Praziquantel) you treat with Praziquantel again, do more water changes and treat with Levamisole. Repeat process 3 times.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
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How often are you doing water changes and how much water do you change?
Do you gravel clean the substrate when you gravel clean the tank?
How often do you clean the filter and how do you clean it?
Have you added any new fish in the last 2 weeks?
Wipe the inside of the glass down and do a 75% water change and gravel clean the substrate each day for a week. Clean the filter if it hasn't been done in the last 2 weeks.