I finally have my CO2 injection dialed in correctly according to the drop checker in my 55 gallon planted tank and I thought things were straightened out. I tested the tank water today with API liquid test kits - the ph is between 6.4 - 6.8 and my nitrates are reading between 40 and 80 ppm.
I have just ordered 12 amano shrimp to add to my tank and I need to get these water parameters back in balance.
Just to be on the safe side I checked my tap water that I have been using for all my water changes and the ph was almost exactly 7.0 and there are 0 nitrates in my water supply.
I am kind of confused since both my cory catfish and my plattys are breeding as evidenced by the presence of cory eggs on the tank walls and my 4 baby fry plattys. My established fish besides the corys and plattys, my Otos, long fin tetras and Harlequin rasporas are doing fine, however the cardinal tetras I recently added are not doing well. How can this be?

Thanks for any advice and suggestions?