Troubleshooting Quarantine Tank

[Exus]

So, this is my first 10gal Quarantine tank. I cycled it for about a month using media from my Display.

So today I got in 10 clownfish, 1 inch. After getting them settled in, I checked back after lunch after about 2-3 hours, I noticed that the fish were behaving weirdly. Some appearing to try and jump out of the water, others sinking to the bottom gasping. A couple at this time appearing dead.

I did some quicks tests, ammonia 0, nitrite 0, nitrate 0, ph 8.0, salinity 35ppt, 78 degrees F. It has only been a couple hours and I don't think they should have died even if the cycled wasn't complete, the ammonia shouldn't have accumulated too much yet.

So, after not being able to figure out where the issue was, I freaked out and put them into my Display (bad idea in hindsight).

I put all 10 into the tank, hoping maybe they'd recover, but throughout the rest of the day, they died off one after another. I guess maybe it was too late by the time I discovered them. Now, 6 hours later, and all of them are gone.

After thinking, I'm considering maybe oxygen was the issue. It was a 10 gallon tank, and I just had a hang on back filter. Potentially it wasn't sufficient to properly aerate the water?

Anyone have any insight for me?

 

[Jekyl]

10 fish at once in almost any tank is too much. Slow and steady wins the race. I'm guessing ph issue like you said.