I will freeze the fish because I would usually bury them in the backyard during the spring through summer as a fertilizer. And I agree it is the water, but why wouldn't my eel die first, knowing spiny eels are sensitive to water. Tank size is 45 gallons, been set up for over a year. Cycled for 1 month plus. When I change the water, I also vacuum around the tank but mostly in the plants since a lot of old food and fish feces gets in there.
I added my fish one at a time in quarantine first by putting the fish bag in the tank for 20 min, then opening the bag and pouring some of the water from the tank into the bag and wait 20 min, then I release the fish.