Check the water quality for ammonia, nitrite, nitrate and pH.
Do a big water change.
Make sure the tank has some dark substrate so the fish aren't stressed as much.
Make sure the aquarium has a picture or something on the back to make the fish feel more secure.
Make sure the temperature is suitable and there is plenty of aeration/ surface turbulence.
Post pictures of the fish so we can check them for diseases.
Most common livebearers from pet shops carry intestinal worms. If you plan on adding them to the tank with the celestial danios you should probably deworm them first. Personally I would keep the celestial danios away from the other fishes and try to breed them in a single species tank.
If you want to deworm the fish, see section 3 of the following link.
Fish do a stringy white poop for several reasons. 1) Internal Bacterial Infections causes the fish to stop eating, swell up like a balloon, breath heavily at the surface or near a filter outlet, do stringy white poop, and die within 24-48 hours of showing these symptoms. This cannot normally be...
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