I bought some guppies a couple of weeks ago. Since they all came from the same tank and were pretty much the inhabitants of their own tank I didn't bother with a qt
Yesterday morning, before work, I was feeding them and noticed that one of them was at a different angle than the rest. The others were pretty much parallel to the water's surface when eating while he was at a 45 degree angle. He was still eating fine, so I didn't think much of it.
When I got home, he was dead, stuck to the filter intake. They never swim by it so I'm assuming he sank and was pulled by the currents in the water.
Right now I'm just keeping a closer eye on the others. The only symptom I saw was his angle in the water.