When it rains , it pours

So now my third tank which has ocellatus gold and compressiceps and cyprochromis.... now seems something has gone terribly wrong.
The ocellatus look ok but one of the two cyprochromis died and the two male and female large sized commpressiceps are both dying. I immediately put the compressiceps who were literally on their side into the hospital tank with my Williamsi who has been in there a day and a half. I had put Metro in the hospital tank yesterday. By the way a day before the third tank showed signs of problems, I took the nitrate level and it was as high as possible. I immediately changed the water 50% and put in the salt etc. Prior to this I had the guy who made the tank come back and take out a screw and clamp that was rusting into the water. Can that cause this problem? Can that make the nitrates go up? I also got some garlic extreme and put one drop into the 10 gallon hospital tank. I also decided to put the drop in the food of my other fish as a precaution because I am about sick of the dying. Meanwhile the two beautiful male and female conpressiceps 3" large are in the hospital tank lying on their side so I keep swishing the water to make them swim as much as they can and even hold them upright to try to coax them back top life. They do respond a little.
Also with the metro it says to add medicine after the second day but it does not say to change the water before adding.
By the way the third tank now has a nitrate level of 20.