Awuh, I am so sorry to hear that, Jose. I really think shops need to be held accountable for this kind of thing. It's not legal for them to sell visibly sick animals either. Ich is very common and all pet shops should be able to spot it.
Unfortunately, when it moves that fast, it is not easily treated. Most likely your fishes immune systems were lower as well due to the stress of being moved and having new fish added into their home. =( I used to have a platy tank, I'd had it for a long time before I decided I wanted to add in some black and gold mollies(I was told they were the original Liberty Molly, I'd never seen them before), and all of them had some white spots on their black...I THOUGHT it was natural coloring, I had never seen ich before. But it was very visible they had it, and yet the shop still sold them to me. They also sold me a molly with Columnaris and wouldn't take her back because they had no QT tank. =-=;
Well, in anycase, it killed my entire Platy Tank in just four days. My pregnant white MM platy was the last to go. v.v All that I had worked hard on and spent a great deal of money on, gone. I was bummed. I hope it doesn't depress you as bad as it did me. Use it as a learning experience. These things happen, and unfortunatly can wipe it all out in just a couple nights.
I ended up starting a molly tank next, and again had an ich outbreak from a purchased black molly(She had a very, very small spot on her tail I again mistook for natural markings, it was the tip of one end of her tail and that was it, nobody else showed anything like it in there, boy did I feel dumb taking such a risk!), but I researched it this time and was ready! =) I noticed she had a spot on her back the next day, so I turned the temp up to 84(That's the highest mine would go, and it will work above 82 degrees, it's just not as hard hitting to the ich as higher temps are), and added aquarium salt. I actually turned the temp up slowly, she was the only one sick and slightly at that, I didn't want to stress my other fish out by doing it so suddenly and weakening their systems to it, so I turned it up about one and a half degrees per day. My heater hates going up and staying exactly at a specific temp, it's usually in halfies. lol The ich shrank and disappeared off of her by day three and never spread more than it did.
So for future reference, try that. =) 84-85F temps, extra air since hotter water means less oxygen, and aquarium salt. When I get new fish, I add salt in there as is(Half the required dosage for the tank size when nobody is visibly ill), just in case, and my temp is always at 80 degrees. Mollies like it warm, so that makes it easier to ward off parasites since -most- of the freshwater ones hate waters warmer than 75-78 degrees. However, be sure you have a back-up plan, research safe ich medicines as well. The Heat+Salt treatment DOES work, but it's one of those might or might not, but usually, kind of treatments and largely depends on your fishes immune systems and how bad the outbreak is. If your fish has a very low immune systen or went through recent stress/trauma, many things wont work. =(