Have you tested your water? If so, what are the results?
Did you add them all at once? Even if you didn't, 18 rosey reds is majorly overstocking a 10 gallon tank. I wouldn't keep 18 rosys in anything smaller than a 55 gallon tank. Adding them all at once also overloads the filter, and shock results, killing several, if not all, fish.
How often are you feeding your fish? If it's more than once a day, they are probably overfed, another common cause of death in aquarium fish. I also recommend not feeding your fish for the first day or two after you've bought them or they've been moved. It gives them time to adjust to the tank.