You've been in Germany? Hope you ahd a nice time

Wish I could go on holiday lol
210 litres is about 50 gallons - not large enough for any silver sharks. In fact, if the shark is stressed, it may be swimming along the tank very rapidly continuously and this would also stress all the rest of the fish out. Silver/bala sharks are schooling fish, they grow very large and, in a small tank, become very nervous. It's good that you do have them in a group but these fish still need 125 gallons minnimum. I'm sure this isn't the first time you've been lectured like this but it
is realy important that you realise that the silver sharks' distress will have consequences on the rest of the fish as well. I am hoping that the 3 you have are still small and that you are either going to move them to a larger tank ASAP or you will return them to your LFS. BTW, when those sharks are nearer to 12", the neons will almost deffinately get eaten.
If the gourami has injuries, they are either due to the sharks (if they are relatively large - and they aren't aggressive fish so it won't have been done 'on purpose' so to speak) or the other gourami. The fact that you have two females is lucky though as it means they may be ok together indeffinately. I imagine the sunset (dwarf) and banded gouramies are relatively peaceful towards your three-spots - if anything I imagine the three-spots are aggressive towards them?
You say your tank's water parameters are 'well below the acceptable level'. Just to clarify, the acceptable level is 0 ammonia, 0 nitrIte and nitrAtes under 40ppm, prefferably under 20ppm. I don't know what the test kit says but if you have anything above this, do some partial water changes with de-chlorinated water because it may be stressing your fish out and making them prone to illness.
I won't pretend to know what's causing this

I haven't a clue. I'm just trying to establish any possible role various factors played in this. Is the gourami still swimming like that now you've come back? Based on what Majjie said, is your gourami eating well? Does she look 'lethargic' or like she's 'wasting away'? If so, the problem could also be an internal bacterial infection (lack of appetite, when there's no competition from more aggressive fish, is almost always a result of this, if seen in gouramies). A lot fo the fish you have (sharks, other gouramies) could be competing with her for food though and leaving her hungry - make shure that's notthe case and try feeding some sinking foods alongside some flake to distract half the fish and give thios gourami a chance to demonstrate whether or not she eats enough
