HELP!!! Pond Fish Killed My Bala Sharks!!!

[MoscarTheOscar]

Okay me and my brother went and collect some guppies from a retention pond to feed my oscars but while we were down there we caught a larger fish which we believed to be a baby mullet. We added him to a different tank containing two Bala Sharks rather then the oscar tank to be slaughtered with the small guppies. After adding him to the Bala Shark tank we realized he was a Killifish. With in about and hour-hour and a half one of the sharks started swimming vertically and not moving his fins normally(almost as though he was having oxygen deprivation or spasms). He started breathing unevenly and vibrating like a phone. In about two minuets from the uncontrolled breathing he shot around the tank like a bullet then just floated to the top. Instantly after he died the second bala shark started with the same syndromes. I moved him to a medical tank(new water) to hopefully save him. He lasted longer than the other bala but eventually died as well. The second bala shark did not shoot around the tank at the end of his run though like the first bala shark did. The killifish is still in the original tank moving and breathing perfectly fine as though he was never moved from the pond. I have absolutely no idea as to what happened I tried googling diseases and parasites caused from wild fish and from pesticides but returned with nothing. Please if anyone knows what happened reply to this I am so lost and really want to know what happened.