Help, fish death!

[syncro]

Thanks for the ideas daniella, Aquattro - good things to check.

I don't think it is an oxygen, salinity or overstocking problem.

- Temp is 24.5*C +/- 0.2*C.
- Skimmer running 24/7.
- Salinity is high, but stable. It increased over the course of a year - caused by evaporating refractometer calibration fluid.
- Lightly stocked and healthy: 3 cardinals, 2 clowns, inverts in 40g. Two weeks ago the cardinals spawned. I figured the unusual behavor was from the spawning.

Today a second cardinalfish is sick. His mouth turned white with a few short strings of white hanging off. Not sure if the strings are growth or his skin deteriorating. Tonight his mouth has turned red. Best guess is cotton mouth bacterial infection. Moved both surviving cardinals to hospital tank and started Maracyn 2 treatment.

Thoughts? Thanks for the help!