Am I gassing my fish?

I am not new to keeping fish but I am new to keeping a planted tank.

I have a 33 gallon tank with pressurized Co2 that I did a Dry-Start for 8 weeks or so. Then I planted it and let it cycle for another 8 weeks. I am doing EI dosing with a 50% weekly Water Change. I had some initial algae issues but I cleared that up by reducing light, spot treating with Excel, and adding 12 Cherry Shrimp, 4 Ottos, and 12 Nerite snails.

I had some initial die off with the shrimp and Ottos, but I didn't think too much about that as the tank was still settling in.

But then 2 weeks ago I was at PetSmart getting something for the dog and saw they had Cardinal Tetras on sale for $1 each. I thought it was about time I put a nice school of fish in the tank so I bought the last 14 they had. They all seemed full grown so I took them home and gave them an acclimation period of about 1.5 hours where every 15 min or so I would put in about 1/4 cup of tank water in their bag. When I first added them to the tank I had the lights off and the only issue I had was the Tetras all stayed at the top gasping. I assumed it was because they came from the store right to my Co2 injected tank ....as about an hour later they seemed fine and were swimming near the bottom and middle of the tank.

Everything seemed fine for the first few days but then I found some dead tetras one morning. I didn't think too much of it as I expected some loss. But then a few days later I found another dead Tetra.....And then another. And this morning I found another dead one stuck to my Korilla Nano and now just a few minutes ago I found another. I am down to 6 live Cardinal Tetras, 2 ottos, 6 Shrimp.....all the Nerite snails and hitch-hiker pond snails are still going strong (go figure).

The water parameters should be fine for these fish best I can tell. Ammonia=0 Nitrites=0 Nitrates=30~40ppm PH=6 to 6.4 (depending on how long Co2 has been on) KH=2 GH=4 Temp=26C

The only thing I can think of is that either these fish were all old when I bought them (maybe that is why they were on sale for a buck a piece) or I am gassing them to death with the Co2. I have a drop checker and I use some pre-mixed 30ppm solution from SuMo and my 4dkh drop checker stays green all the time, even after Co2 being off all night with an air stone on. I assume the solution is good because it is dark blue until it has been in the water for about an hour.

Anybody have any suggestions as to what I should be looking for?

I don't want to spend money on more fish until I get this straightened out. It is frustrating because my low-tech (heck it is no-tech) cheepy 10 gallon Betta tank is doing fine with 1 Betta, 4 White cloud minnows, and 1 Otto. All it has is the stock light/top, a 15 watt heater and a Whisper 20 filter. I spent hardly any money on that tank and it seems fine.....while my high-dollar high-tech show-piece tank seems to be a death sentence for my fauna !!

Thanks !

Bill321


Here is pic from a few days ago. I think the Nerite snails are having an orgy in the bottom right corner.