Have you added salt to the tank?
You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 2 heaped tablespoon per 20 litres of water. If there is no improvement after 48 hours you can double that dose rate so there is 4 heaped tablespoons of salt per 20 litres.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.
The salt will not affect the beneficial filter bacteria but the higher dose rate will affect some plants. The lower dose rate will not affect plants.
After you use salt and the fish have recovered, you do a 10% water change each day for a week. Then do a 20% water change each day for a week. Then you can do bigger water changes after that.
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It's possible the fish are still suffering from pH and GH shock after being put in the shop tank and then into your tank within 24 hours.
Check the GH and raise it up to 250-300ppm. The harder water helps reduce bacterial infections and the minerals help to bind to any poisons that might be in the water. Make sure the pH is above 7.0.
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Generally when rainbowfish stop eating, go pale, do stringy white poop, and sit under the surface or near a filter outlet, and die a short time later, it is an internal bacterial infection like Tuberculosis (TB). However, I doubt this is the case with your fish.
Check the water and make sure it's all good and add salt.
Don't do too many water changes just now. If the water is good, don't do a water change for a week or two. This will give the fish time to adjust to the tank conditions.
