strange guppy disease

#1
Hi everyone! Last summer, I lost about 15 adult guppies and some fry to a mysterious disease that only affected my guppies (the danios and tetras were unaffected). I'm setting up a new tank now and I really don't want to lose my guppies again, but I have no idea what caused it or what it even was. If anyone knows what this is and how to prevent it, it would be much appreciated.

The fish swam in an odd manner, taking on a sort of s-shape with their bodies. Their dorsal fins appeared to have disappeared. They may have just been flattened down. Once a fish looked this way, it died within three days. The bodies turned black, starting at the tail-end and then working its way towards the head until either the whole fish was black or the fish died. As the fish came closer to death, they stopped trying to swim sank to the bottom of the tank where they usually stayed and died. The dead bodies never floated to the surface.