I agree with Bandit that Columnaris is likely responsible for many sudden unexplainable deaths in the aquarium. It seems to rot the fish from the inside first and by the time the outward symptoms are visible, it's too late.
Potluck, in your Rainbows case, considering he is swimming normally and eating, I can't help but think his sudden onslaught of ailments are from a weakened immune system due to his advanced age. Just remember we all have these obstacles to overcome.
Full disclosure, 3 out of 4 of my juvenile Red Texas Cichlids I bought 3 weeks ago suddenly died. No outward symptoms. Just stopped eating and passed the next day. Went back to the LFS and found that the remaining Red Texas Cichlids looked unhealthy and not many left. I believe Columnaris to be the bug. $16 bucks a peice. Haven't had an outbreak in years.
For what it's worth, when an outbreak does occur all I do to treat is more frequent, larger WC's.
It seems to work as well as the meds I have tried. The last med I used was the old standard, Formalin. Great for parasites. I've had zero success with any meds reference bacterial infections.
I hate that word, I lost 29 fish in 3 weeks about 4 years ago, I paid $16.00 for my rainbows as well, its not really the money it's I keep my tank immaculately clean(besides my intake tubes cause i always forget to clean them lol) And my water parameters are always 100% perfect, he's older, now he's just swimming on one side of the tank by himself but gets excited for food and eats healthy, I did gradually lower my tank temp to 75 because bacteria grows/spreads slower below 76, I can't do 74 because my house is always 75 (sucks but I have elderly people that like it warmer).
I don't know weather it's best to euthanize him In case it is columanaris that way it can't spread through my whole tank unless it's like ich and already contaminated the tank, or to bomb the tank with malechite /formalin, or furan 2 and the other one that you use with it when treating for columanaris, I'm not very good with what to use for what disease /illness as I haven't really had many, columanaris once but after all my fish died I took a few dead fish into an advanced lfs that did scrapings and said it was, then I had 2 angels die a year later from a fungus no one could help me with and after I did research all I needed to do was add formalin/malechite blue I think they said and ich and that's it besides a few fish that got bloat even I first started because I had crap filters no test kit and very poor water quality.
I guess I have just been really bummed since my clowns died from the ich, I do 35% weekly now instead of 30%, so I'm just looking for what I need to do to save my fish, what meds, do I add salt, etc etc I don't know what it is, his eye was perfectly fine 15 hours ago, it looks like there's a protective lens over it or something, I'm just not good at diagnosing things like this cause there's so many things that cause the same outward symptoms.
Here are my water Parameters again as I just checked them
Ammonia 0
Nitrite 0
Nitrate 10 (dropped down cause my water change)
Kh 6
Gh 6
Ph 7.5
Phosphate 1.6