Pygmy cories swimming upside down and dying
This is my first time owning pygmy cories, and I got three from a lfs. They were doing perfectly fine for two weeks, very active and playful in my 10 gallon with small school of tetras. The plan was to get a few, see how they did, and then get some more after my school loans are paid off at the end of the year.
At the two week mark, two of the three started swimming weakly, erratically, and upside down, and ended up passing within a day. I talked to the shop owner, and he said he'd never heard of that happening with them. I chalked it up to some sort of unknown illness, but the third one just started acting the same way a week and a half after the other two passed and I don't have high hopes for his survival.
The tank is at about 72 degrees (I didn't know I needed a heater with pygmies, I'm planning to get one after I do my next loan payment) and water conditions are perfect.
Is there something I'm missing? I really love my cories and I'd hate to be doing something wrong that's killing them off by mistake.