Water is Good But the Fish are Dead/Dying

[JarettRoss]

I have a 16 Gallon BioCube tank that is now roughly 8 months old.

I have done multiple water tests in the last three weeks and they have all been completely fine/good results, including a spin test.
In the last three weeks I’ve lost three very stable fish that have been in the tank for months. I have multiple corals that are doing fine as well. I cannot understand why the fish are dying off and what to do.

 

[blaxsun]

You'd need to first list your water parameters, ie: ammonia, nitrites, nitrates, phosphates, salinity, temperature, etc.

 

[Sharkbait19]

As said, listing the actual numbers of water parameters would be very helpful - you may be missing something.
Pics of the fish before death as well as detailed descriptions of how they looked and acted beforehand is most important for diagnosis. Even pics of the tank will be helpful.
Also some general questions that will help a lot:
What kind of fish?
Any recent additions?
Were new fish quarantined?
Has there been any pathway for chemicals to get into the tank?

 

[LeftyReefer]

If water parameters are good, then dead fish usually means sickness/disease/parasites.

Was anything (fish or coral) added recently that might have brought something in?

did you see any of the fish acting differently before they died? fast breathing? anything?