Sorry to post again. chromide problem

My fish were sick a week or so ago and they are now recovered except for the orange chromide who either stays in one place and rocks wildly with fins clamped or swims in a jerking motion with fins clamped. He dosen't eat very well, he spits food out. Any suggestions on what might be wrong with him would be apprieciated. Sorry to post, I know I'm a complete idiot but I would like to try and save him.
thanks

I never refused to read any advise

At least that I'm aware of, really, if I did, I'm sorry.
actually faramir hasnt' got back to me yet but here's whats happening and it isn't good.
I tested the water when the problem first started to post the real water paramiters in my post (rather then guess them) then, the ammonia was 0 nitrite was 0 nitrate was 5.0 and the salinity didn't even register on the hydrometer. the gh is 160, the ph is 7.4, didn't test kh. When I saw the problem first I cleaned the whole tank from top to bottom and treated with maracide and maracyn 2 and more salt. The fish got better but.... I think I killed my biofilter. I just checked the water and the nitrite is 0, the nitrate is 0 and the ammonia is .50. The salinity is now .oo1. I guess I blew it
Now. I changed some water, 13 gallons, to get the ammonia down. I am going to put a sponge in from one of my other filters to try and seed my filter again. Please let me know if this is the wrong thing to do.
I have 5 other tanks and never have trouble with those, at least nothing like I have with this one.

I still wonder if theres some parasites.

this is why. There is something sticking out of the gudgeons face there was another one yesterday but it's gone now. IT kind of look like a brownish, 1/8 of an inch long, thickish threads. Also when this happened last time it was about a month and half ago. The fish got better and I lowered the temperature to 77 because it was up at 80 when they were sick. The heater wasn't real accurate and it went down to 75. Within two days the gudgeon was gasping and sick again and it looked very much like the last time. Does this make any sense at all?