Rest In Peace, George!

I was hoping I'd never have to post a thread like this... I woke up this morning feeling pretty ick, and after grabbing something to settle my stomach, I came back to find George laying upside down in the tank, dead. I'd checked the tank and done a head count last night before I went to bed (just after midnight...), and six hours later he was gone. There were no signs of injury, gills appeared intact although there was a tiny patch of fungus I noticed when I took him out, and his cloca had been slightly swollen, which I had thought to be more due to us coming into the breeding period... No floating or other signs of impaction beforehand (though all the stones in the tank were larger than all their heads- and he was the second smallest of them).
The tank itself had a 15% water change last thursday/friday and was due for another today, I added in the canister filter and chiller just over a month ago, and both have been working as far as I know. Nothing else has gone in, no new plants etc. Owing to the lower temperatures, I've been feeding them every second-third day, bloodworm choc-block and beef heart as the worms have all disappeared to the bottom of the compost heap here. There are also some RCS in the tank to supplement their diet should they get hungry. Plants are all live (Though some of them have been looking a bit dead, which would explain the higher nitrate reading). The past week I've upped the chiller "on" point from 13-16 degrees (Which we haven't hit in months), and adjusted the light "on" approx. 1 hr either side of where it was a week ago.

To the nitty gritty details (Taken after I'd got George out...):
Age: 1 year 1-3 months.
Colouration: wildtype
length: 17cm (approx)

Tank
Temp: 13.4
pH: 7.8
Ammonia: <0.25ppm
Nitrite: 0ppm
Nitrate: 80ppm (which I was/am aiming to get down with the water change today).

Any thoughts on if I should move the others out to be on the safe side, or even COD?

I'm sorry for your loss How very sad.

Oh Caitie, noooo

Sorry to hear this.

Other than the high nitrate I don't know what to suggest? The others are all ok and well?

Could anything nasty have got into the water?

Others are alive... Peta's tail has been red for a few weeks now (from the search engine, a sign breeding season is near...?) As to contaminants, I've been pretty careful (moreso than usual), nothing gets sprayed in there, my hands are always washed before they go in, the water I use is tank water with the salts then added in (And a TINY bit of pH buffers)...I wonder if there was some sort of contaminant in their food which in and of itself would be worrying...
Thanks for your support