My fish are drastically dying. please help!?

I had fishes for 5 years already sometimes few would die after they grow old and everything.. but today i come home after i left for 2 hours and about 10 of my fishes are dead all of a sudden and i do not know why.... so i put them in a different container with cleaner and more cooler water and yet they still continue to die... another 10 died after i put them in new water. The fishes are Red Wag Swordtails. I dont know what to do I really need your help, please help me!