So...in the past 48 hours, I have had 3 guppies and a Cory catfish die. I bought the guppies 10 days ago and the Cory 2 days ago. Nitrites-0, ammonia-0, nitrates-5, ph-7.6, temp-78. I tested last night after the most recent deaths. The only changes have been that I added driftwood. My other fish are acting perfectly normal. Did I just get bad stock? Any ideas? I need to know if I'm doing something wrong before I replace them. Thanks in advance!