Ok,

So things have stabilized. I got a heater and now I'm running an even 70F at all times. The 6 fish I still have appear to be much calmer and they has stopped dying.

Thanks guys for all your help.

I don't think that I have a chloramine issue. I've been treating for that with vitamin C when I add water, and because my ph is running high, I've been being generous with the vitamin C.

I've done some more reading on the subject of koi and temperatures and it seems that they have 2 different immune systems. A cold water immune system which is non-adaptive and a warm water immune system which is able to adapt to pathogens and is much much more effective at beating disease. It seems that it is about 58F where the warm water immune system switches on or off, and I was dancing back and forth across that temp virtually every day. Perhaps that was the issue?

Anyway, now I've got a system that is running with almost no fish in it, and so the ammonia is not keeping up with the plants.

Ph = 8.0 (trying to bring this down slowly)
Ammonia = 0ppm
Nitrite = 0ppm
Nitrate = 0ppm

Ph clearly too high. I don't know why it has creeped up? Perhaps they warmer water? I also noticed that it started to fall as the bacterial colony grew. I expect that with so little activity in the tank, the bacteria are starting to die off and this may be why the ph is climbing again?

So, it seems like I ought to get some more fish... but I am weary.

Even though the fish I have are not dying anymore, I'm not confident that they are not just super tough carriers of some disease cocktail. This experience has made me aware of the idea of a hospital/quarantine tank, but at this point I'm not 100% sure what I would be quarantining from what... am I protecting the old fish from the new fish, or the new fish from the old fish?

Also, what is proper quarantine method? On one side, I'm tempted to start an entirely new system and cycle it for my new fish, so that I will know that what ever pathogens they have are the ones they came with. Of course, the advice for cycling a system is to take some media from an already cycled system... so there is a cross contamination. On the other side, the new and old fish are all supposed to live together in the end, and the old fish are still pretty small, so what is the quarantine really going to do? Also, if I don't get a bigger ammonia source into my first tank soon, then I figure I'll start loosing my plants.

I'm not really sure how best to move forward from here. Got any suggestions?

Thanks,

Bill