Diaphus sp.

You have some wonderful fishes from Poland, Thomas!

Diaphus  is in the same order as Eomyctophum, right?  Myctophiformes? 

I can see some similarities between them. 

Great fossil!

 Yep, family Myctophidae and order Myctophiformes.

But you have to be very careful. Some slabs - especially the very dark slabs from Jamna Dolna - are prone to pyrite desease / decay. If you can not keep the relative humidity below 40% (better 30%!), they start to rot and in a couple of years, they will look like this slab:

After some painful losses I keep all my fossils from Poland in small plastic boxes with silica gel bags and some ammonium carbonate. Ammonium carbonate slowly degrades to gaseous ammonia and carbon dioxide - the Ammonia neutralizes the sulfuric acid that is formed by oxidation of pyrite and stops (or at least retards) the autocatalytic effect.

You mentioned this once before, about the susceptibility of the darker shales to pyrite disease. 

Mine are kept in a cabinet in a dry area with silica gel bags.