Ammonia levels between 0.5-0, and my gourami has gone a bit clumsy. It’s floating up to get air occasional but staying at the bottom and kind of floating about, that one and the male gourami with her have both had black stuff on them which is really visible today, and the female as a striking white line on her fin/back, what could this be? It’s not looking good for her I’ve added melafix anyway but want to prevent further sick fish.
The best you can do for her at this moment in time is to get/keep water at 0 ammonia, 0 nitrite and <20 nitrates (or no more than 20 above tap water nitrate level). Black is often a sign of ammonia burns.
I'd also ensure that you feed some veg, just in case there's a blockage internally that accounts for the floatiness. Take the shell off a pea, cut each half into tiny morsels, and feed that.
She possibly looks slightly bloated on one side - this doesn't necessarily lead on to anything but I'd monitor her closely. Do you, by any chance, have eSHa 2000 or Waterlife Myxazin to hand? If the situation deteriorates, especially if her scales stand up like a pinecone, then it may be worth treating with that. Melafix is fine (I've used it successfully) but, with labyrinth fish such as gourami, you need to be all the more careful *not* to overdose - err on the side of underdosing if at all possible.