Molly fish with hunched back? Or bent back?

Colin_T said:

Doesn't look like either but better pictures taken under white light might offer more information.

Check your water quality for ammonia, nitrite, nitrate and pH.

Find out what the GH (general hardness) and KH (carbonate hardness) of your water is. You can call your water supply company for this or check their website. If they can't help, take a glass of tap water to a pet shop and get them to test the GH and KH. Write the results down in numbers and find out what the test was measured in (eg: ppm, dGH or something else).

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If you ever have mollies, guppies, platies or swordtails that appear off colour, do a 75% water change and gravel clean the substrate every day for a week, and add some salt.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

And add 2 heaped tablespoons of rock salt (aquarium salt) for every 20 litres (5 gallons) of water. Keep salt in the tank for 2 weeks.

And post pictures so we can check them for diseases.

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