Actually, if we go with Nate's suggestion about poor oxygen exchange (which I would agree with btw) this could explain the heavy breathing and death. It could be that your fish were just barely getting by with the poor oxygen level in the tank. Then one day you add a powerhead which introduces more flow which forces the fish to swim more which causes them to breath heavier consuming more precious oxygen which is already lacking in the tank and releasing deadly CO2 and suddenly you have a series of unfortunate events :(

I agree, I think I'll set up the skimmer and once the film isnt present ill start the powerheads up, thanks for all the advice, I'll update as the whole situation unfolds:biggrin:

You could try throwing an airstone in there on an air pump until the skimmer is all setup and running.

how old is the aquarium? if it is old, it could be the "old tank syndrom" as you added a power head and it might have steared underneat the sand and released some toxic gas?

That is a very good assumption because it is almost two years old now, but how can I prevent that from happening again when I put them back in.

Slowly siphon the sand bed and ensure you are stirring up the deeper sand. In theory it should suck up and remove any gasses.

I run 5 Koralias in two tanks. Never heard of a coating issue. With any brand for that matter.

Good luck.

I'm thinking a little different to most on here. Did you was your hands before you stuck them in there? Could be a poisoning thing going on in there?

Ya I always make sure my hands are clean, I don't wash with soap though but I always take all the precautions.

You and I both know that the chances are very likely that the powerheads caused the issue... I had the exact same symptoms in exactly the same duration of time after adding the exact same brand and model of powerheads.

I ended up losing my whole stock after this episode brought on a major case of velvet from the stress caused. There was no evidence of any ich or velvet outbreak the day before. It would be impossible for it to take out 15 fish with all the same symptoms in a night.

Wash those powerheads in vinegar and water for many hours before using them again. If you notice any onset of ich due to the stress caused from this episode I would quarantine right away... Dont wait and try try to ride out like I did.