I purchase 4 guppies last week and they all died with in a few days of each other. I returned them a got replacement for the 4 I lost and purchased 2 additional guppies. Now less then a week later 3 of them died. There are no signs of attack, chasing or nipping no injury at all. Water condition etc. I have a 65 gal planted tank. The PH is 7.2 with a CO2 injected. My ammonias are 0-.25. Nitrates and Nitrites are all in normal range. The KH 3-4, GH 10-12. Temp 78 degrees. I do a 50% water change weekly for the past several years. So all my water parameters are in great shape. I have 12 neon tetras, 5 black tetras, 10 glow tetras, 6 platies, 2 algae eaters, 2 cory cats, 1 male molly and 1 horn snail. The all live in peace and are healthy. I have raised guppies since I was 8 years old and never had a problem until now. Please advise a expert on what you think may be the problem.