Aquarium Trauma

Major tank die off of fish and we don’t know why. Water’s right and the temp is right. The fish don’t even look sick until they suddenly stop and die. Nothing weird looking about them. No discolorations. No swellings. No rot. This began a week ago and has hit critical mass today. We’ve not a clue what is wrong and no information we get is any help. We’ve lost some of our favorite guppies.