My clownfish died just after a day

I have a 6 gallon nano tank and wanted to make it for one clownfish, It had pool sand and the water was fresh sea water. The temperature was 76 degrees and there was some hermit crabs and shrimps. The filtration was 435lph internal filter with biomedia. The filter and the tank was new. I know I rushed things but I did a full water change later that day when I saw the clownfish staying at one spot and was my intention to keep doing partial water changes untill the nitrogen cycle was complete. Later he was laying on his side breathing very fast. I didn't know what to do next. The day after I woke up and found him dead. I still could not understand how he died because the other creatures were fine and are still fine now. I don't think that the ammonia would spike that quickly to kill him.

Any thoughts how he might have died?

 

"Until the nitrogen cycle was complete"

Your cycle wasn't finished? Water changes during the cycle are not beneficial. Partially defeats the purpose of it.

 

Could it be the sand? maybe it altered the ph or something.

 

Test your ammonia just to be sure. Being that everything else is still fine though, my first guess is that it may have had brooklynella, aka clownfish disease. Everything else in the tank is far more sensitive to pH swings than a clownfish.

 

Could the clownfish died of high salinity like 1.028?

 

Yes if it came from a low salinity system. It's certainly would create more stress during acclimation than the fish might be able to handle.

 

1.028 is pretty darn high. High salinity will lower disolved O2. Do you have anything in the tank for circulation like a power head or skimmer? I would lower your salinity to 1.024. Also, you might need to buy a test kit to test for ammonia, nitrite and nitrate to make sure the cycle has finished.

What is pool sand?

 

pool sand is a medium grain silica sand. I'm guessing it is pool filter sand.

 

The marine aquarium is very nice to own but require a lot of time and money to run it. It's not for me for sure.

 

^^^^