Hi, sorry to hear your fish aren't doing well. Im relatively new here, so I may be missing some info if you've posted it elsewhere. Could you tell me - how many fish/what type, no. of gallons, what kind of filtration/aeration ?
Is the water as white and cloudy as it looks in the pics? Have you looked at pics of ich (aka white spot disease) - do your fish look like that? How do the gills look - nice and pink (good) or puffy and red(bad)?
Even if the fish have a specific disease like ich or gill flukes (problem I was having for a while), factors like poor water quality and over crowding would have be addressed along with doing the chemical treatement - those things can weaken a fish's immune system and make it more susceptible to disease. You could have several things going on at once. I would not get anymore fish until you get this sorted out.
BTw, there are things like dissolved organic compounds that can build up and are harmful to fish, and which wont show up in a water test. Have you done any water changes? You can in fact do pretty substantial water changes over a period of several days and it wont harm your beneficial bacteria since these live mostly in your filter media, sides of pond, tubing etc. (Of course any water added would have to be treated with a good dechlor like Prime or well water free of chlorine/chloramine.)
Anyway it would be helpful to have more info so we can get a better idea of what's going on. Otherwise its just guessing. Thanks, Catja in WI
