I have lost 3 platys at different times to a strange sickness. Their back started curving, but they were still lively and active. However, as their spine started looking more curved, they became less lively. This lasted probably a month or so. They eventually just became very lethargic and passed away. I was really sad about it, because some of them I have had for a year and I was pretty attached to them.
I didn't test the ammonia levels when they were sick. After the third one died, I started testing the water. The ammonia was 0 and ph was 7.2.
I had never tested the water before because the pet store people told me it wasn't necessary. However, now I know I can't really trust the pet store because they told me I could add 6 fish to a 10 gallon after only running the tank for 3 days. Well, it was a disaster. Now I know the tank should have been cycling for a month and 6 fish added at the same time would cause a huge ammonia spike. Also, that's when the first two platys got the spine curving problem.
Strangely, though, the last platy that died had it when the tank was cycled for a year (however, I hadn't started testing the water yet) so I'm not sure why he got it.
Anyway, sorry for the long post. Any ideas of why the died would be greatly appreciated. I just don't want to lose anymore of them!