I have a sick guppy...?

I just got 5 guppies yesterday. They were all doing fine and today i noticed that one of them is just laying on the bottom of the tank in a corner alone. I dont know if its a male or female but its alive because it swims off when i the net gets near it. I have neon tetras in the tank also. I had 4 but one had to be euthanized today because i think it had neon tetra disease. It was swimming abnormally and then didnt swim much at all. The rest of the guppies are ok. Two of them like to stay floating at the top.