Depending on the water parameters of the store's water and your own, 20-30 minutes may not be enough and could have caused shock issues such as pH shock or osmoregulatory shock. Or the fish could have been weak to begin with. I always test a stores water first to see what their pH, GH and KH are compared to my own water so I'll have a better idea of how long to do the acclimation... and often I will still do a trickle acclimation over the course of a couple of hours just to be sure.
The bad news is that if it's pH shock or osmo. shock, this usually only is deadly in the first few hours or so and any survivors should be OK. Did you see the fish acting really lethargic, gasping, etc. after adding them to your tank?
Also, you should have let the tank run for more than 4 hours so that the temperature could stabilize and the water could outgas/ingas to stabilize. In post #44, I specifically said, "Just do the 90% PWC with dechlored water and let the tank run for half a day or so to let the water outgas/ingas and the temperature stabilize and then go get your fish... and you're ready for a full bioload of fish, not just a few at a time."
Half a day is 12 hours, not 4. "Half a day or so" means around a half a day... maybe give or take an hour or so.. but not 8 hours. I'm not saying this is the problem but it could be... depending on what your tap water baseline numbers are.
As far as your nitrite reading, I'm not sure why this would happen... although it could also be related to your tap water baseline numbers... if you have really low O2 levels and really high CO2 levels in your tap water. This would put a damper on your N-bacteria until the CO2 outgasses and O2 ingasses into the water, then the N-bacteria would be back to full strength. You can add a pinch or two of salt per 10G (diluted in some water first) and that will help protect the fish from nitrite poisoning up to 1.0ppm. Do PWC's to keep them close to 1ppm until your N-bacteria recover... presuming they were shocked also. I'm confused about your comment about nitrites being 1-2ppm or off the charts? My API nitrite color card goes up to 5ppm and that color is vastly different from 1ppm and 2ppm... even 1ppm and 2ppm are different to me.
Did you pre-treat your water change water with your dechlor product before adding it into the tank? Or if you didn't, did you at least add the dechlor to the tank first and then slowly start filling the tank so that the chlorinated water didn't start do harm your N-bacteria? I know they are pretty tough guys from what Dr. Tim says but if chlorinated/chloraminated water is on them for too long, it will probably weaken/kill some of them.
