Taking this back a step, this all supposedly started after adding some Tonga rock, mushroom rock, anemone, and a 5 year old chiller. Pull all of them out, as they are the one change you made before the mass extinction. Personally, my money is on the old chiller, something leaching or leaking into the return water. The rocks and anemone are not likely the issue, but you have to start somewhere and a tank reset to its prior condition would be my first step. Once they're out (hate to harp on it without proof yet, but certainly the chiller) see if there's any further death. Do some large water changes on the assumption that a toxin is likely in the tank. Add a poly filter to pick up any funky organics or heavy metals.
I doubt stray current is the issue. Stray, very low level current flow is possibly associated with chronic diseases in fish, especially HLLE. It does not electrocute fish. If there were a sudden current flow though to ground, they would all be dead (there's no fish more resistant to electrocution than another). Also, if there was a potentially significant current potential in the tank seeking ground when you stuck your hands in, you probably would be posting here from the grave, which I would find slightly creepy.
The issue of stray current in a tank is somewhat blurry. I think a more precise way to think about it is current potential (voltage). If there is no path to ground, there will be no flow just because of “current potential”. A live wire cannot just shoot out electrons. It needs to complete a circuit to somewhere, usually ground. When you stick your hands in you may precipitate a current flow to ground, with you as part of the circuit, taking the route of least resistance, which may very well include passing through your fish. This is why GFCI is so important, not a grounding probe, which just converts the potential into a current flow, usually of low amperage. Inductive motors like a typical power head, functioning normally, by definition also create an electromagnetic field potential. That's how a sealed motor can spin a magnet on the impeller shaft.
Getting back on topic, you've placed an old chiller in your system. It might even be older than 5 years, depending on how and when the prior owner obtained it. Chillers have evolved, and continue to evolve to this day. New refrigerants, new alloys, are all being employed to improve efficiency and by regulations banning fluorocarbons as a refrigerant. So, depending on its original construction, I can think of many scenarios (failed seals, material buildup on interior of coils, saltwater corrosion of coils, compressors leaking oils, a long list so throw in anything else you think of) which might poison your tank.
Also, how is your chiller circulating? Are you using an old pump that came with it? Did you use a new pump? Is the plumbing original or did you replace it with new? Any metal hose clamps underwater? Maybe the source lies in association with the chiller, rather than the chiller itself.
Hopefully the source for your losses will come out over time. But, at least remove whatever you put in that was added right before the fish died.
I'm sorry for your loss. Losing those beautiful fish is painful.
I hope some of this helps.
Henry
J/K
