I recently purchased 3 mail fancy tail guppies and they are in a 20 gal tank with 2 platys. I thought for sure that one died because he was at the top of the tank being very still and almost vertical - tail down. I kept watching and was about to get the net and he swam away. He swims for a bit and then floats at the top again (the other two like to float at the top but they aren't as still or as vertical). The first thing I did was check the water which had 0 ppm NH3, 0ppm NO2, 10 ppm NO3, pH 7.4, temp 77.4 F. He doesn't otherwise look ill, no spots, no gulping for air, his back does look a little more humped or rounded than the other 2 though. I've never had guppies before so I don't know much about their behavior. Any ideas?? Thanks, Lisa