Lucy, bent spine and ragged fins are a sign of Fish Tuberculosis....a serious disease that is virtually incurable. Look at the store or on your fish for any open red sores. It is also transmittable to humans, so don't reach into the tank with any open cuts or sores on your hands or you could get major tissue damage on your own hands.
Fish TB is rare in guppies, but since you said "many of the fish look like this" i would go check the store and look for any pictures of fish like this, and definately don't buy any more from them.
https://www.google.com/search?q=pic...z-sMcCFcvTgAod4PsAPA&biw=1920&bih=955#imgrc=_
Don't ever buy sick guppies, because once they get sick they die easily and medicine is not the best approach although it does pay to keep something for parasites like malachite green and or formalin, and an antibiotic on hand.
Melafix is not great but has thearaputic properties. If you want to get antibiotics in the future, look for an online site that has antibiotic food. ALways better to put antibiotics in the fishes food, in the water it does not help much. You must also understand the diffrence between Gram + and Gram - bacteria, and also aerobic or anerobic to understand bacteria in fish, and how its brought on by stress poor diet or overcrowding or poor water quality. Good Luck