Dead Fish - Why?

[Ron99]

Well, it was an unhappy morning for me. I have a 10 gallon I set up around Christmas. It had only live rock, soft corals and the CUC until the weekend. Went and picked up an orange spot shrimp goby, a royal gramma and a clown goby on Friday night. All were acclimated properly and seemed to be doing well for 2 days. They were moving around the tank and even eating a bit. Then woke up yesterday morning to find them all dead.

I am hoping for some insight into what might have killed them all. They didn't have any spots that looked like ick or velvet etc. All the corals and inverts look fine. Just the fish are dead.

My only idea is that maybe the oxygen levels dropped too much at night when all the photosynthesis stopped and everything in the tank was using oxygen up. I have good water circulation with a koralia nano and a HOB aquaclear filter splashing water back in which I assumed would be enough water movement to keep the water oxygenated. Salinity was 1.026 and ammonia, nitrate and nitrites were all 0. pH is about 8.2.

I don't want to get any fish again until I have some idea what might have done this. Is there any possible contamination in the tank that would specifically kill the fish and leave the inverts unaffected?

Thanks in advance for any insights,

Ron