Uh-oh, that sounds very, very bad. I'm so sorry for you and all the fish that have died
Now I'm just desperately trying to think of some possible reasons for this desaster. Let's go through again:
If you have replaced filters and heaters and everything that needs electrical power, it probably can't be an electrical current. My husband has a thing called ampere-meter to check for electrical currents, it is used by electricians. Do you know anyone who may have one?
Else I'd think it must be something highly toxic.
Have you replaced all the water since you took the crushed coral out? If not, could it be that the toxins that may have come from the coral or were just lying dormant in the ground were stirred up when you removed it, causing things to worsen?
Did you treat your fish with copper in the last weeks or even months? Did you mix different medications? BTW, how long has the tank been running? Maybe you posted it, but I couldn't find out. And what exactly is a power surge (I don't know the word surge)?
If you have done a 100% water change or several really big water changes since then, I'd think it wasn't in the coral.
Is the tank all-glass? If not and it is an older tank, could it be something dissolving from the frame due to age, perhaps something metallical (which would be toxic)?
If it's not the tank itself, it can still be something in the water or in the air that is pumped into the tank.
Have you done water changes to your other tanks since the fish in this tank got sick? If yes, it can't be the tap water either, since the others are doing well.
Is the tank in the same room as the others are? If not, have you recently used pestizides, herbizides or any kind of spray? Do you have used a new pan or fondue set or anything containing teflon? A new floor? Anything new?
If the other tanks are in the same room, it shouldn't be anything in the air.
And now I run out of ideas. Perhaps anybody else has a better idea.
If you simply can't find any (other) possible reason, I'd empty the tank completely and set it up new. Perhaps desinfect it before. It wouldn't make no sense to put fish into it just to see them die.
Sorry, no miracle ...
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Gisela
moppel@gismo.owl.de