Yesterday I bought three male guppies for my 10g hex. That night one of them was staying near the surface, not really swimming around much. The next morning two of the three were dead. The params were 0,0,10, and my betta and cories had lived in there fine without any problems. Could the guppies have died from the shock of a new tank? I acclimated them for 2 hours using the technique in the articles page. What did I do wrong? The last one is looking healthy and lively.
Guppies are probably one of the most imbred species of fish. Because of this, many of the guppies you get from the lfs has weakened imune system and the shock of introduction into a new tank can be dramatic. This may have been why your died.