I'm confused as well...i just don't know.
Heater is isolated in an inner filter.
Anyway, thank You for your comments.
I'll use ESHA2000 - just in hope that other zebras won't get anything.

I was watching him for several days and these spots (holes) were going bigger. The day he died, i saw him sitting on the plant leaf & gasping air. Approx.after an hour he was dead. So, he wasn't found at night by other fish.
Could it be something bacterial maybe?

I need your help! It`s a sad day as one of my zebras died:(. I have had zebras already 2 years, but to be honest, i`m not a specialist. Til last days everything was fine. Then i suddenly noticed that one zebra is not hiding anymore. I took a closer look & saw some pink spots on his head. I have ...