Your tank is wonderful and the water looks fantastic, optically speaking. However, your salinity is too low for many of your brackish water fish, in fact some of the fish you have will require close to full strength sea water when adults.
As others have pointed out, you have some fish that don't tolerate salt very well or hardness, severum, blood parrot - and the Africans though appreciative of the low salinity and hardness in some sense, are crowded.
In short, I think you may be over stocked and improperly stocked.
You have a fantastic tank with a fantastic water maintenance routine, why not cut down the numbers of fish, and only keep those who thrive in a brackish environment, with an thought towards the fact that you may need to go to full strength sea water?