Okay, first an update, then a little clarification.
Amazingly, the one gudgeon is still alive. I'm even cautiously optimistic that he may be doing better. He seems to be swimming a little better, and doesn't seem so icky looking.
Here are some pics, for you to decide if I'm just trying to see things.
I also did 50% WCs on both of those tanks.
Now the clarification.
The whole gudgeon thing is a tad confusing, and in my haste, I didn't explain it really well.
I have 5 gudgeons currently.
Gudgeons 1 thru 3 came from Mgamer, and have been in the ten gallon tank since early Dec. Gudgeons 4 and 5 were a surprise gift from a friend, from a pet store in TN. They were put in the ten gallon tank last Friday.
Gudgeon #5 went missing the day after I put him into the tank, and as mentioned earlier, was presumed dead.
Yesterday morning, #5 reappeared, covered in stringy white stuff. Guppies were dead or at the bottom, gudgeons were at the top, pop eye on some of the guppies, etc etc etc.
I did the stuff listed previously, moving #5 to the QT tank, brother doing WCs, him adding erythro, etc.
I came home, and #5, who was in the QT, looked 110% better. However, #4, the OTHER gudgeon from TN, was messed up now. I switched them, and did another WC.
So, I think whatever it was that they had, either the erythro, the salt, or the WC in the QT tank did a LOT of good, as it completely turned around #5.
#4 is the one pictured above, who is still in the QT tank, being treated with salt and erythro.