Cool, I dated a guy from Frankfurt. Only been to Munich though. Beautiful there.
Anyway. The hydrometer is not a great tool for salinity. If you use it, make sure to follow the directions exactly to get a good reading. I honestly never use them. The refractometers are a bit expensive. $50 range here in USA.
Salt is what you want in your tank. I use Instant Ocean Reef Crystals currently with no issues. I have used others too. Distilled water from where? Do you buy it that way? If so, you may want to invest in a RO/DI unit to make your own water. These will run you about $150 USA and then filters annually $80 at most.
I cannot see the fish close enough to know if it is dealing with ich, but treating it in the tank with corals is not a good option.