Think I messed up and took so really bad advise. After adding 3 new fish to my pond I was talking to lady at fish store and she said even tho they seemed fine in quarantine they could still have flukes. Said just add it to pond since they are already in it. Everything seemed fine until 5th day which was yesterday I can home to one of my fish I had for years dead and another gulping at surface. All of them seemed slugish. I immediately checked water and had 0 ammonia 0 nitrite. Never had a fish death from water quality or issue with water quality so I'm assuming its prazipro overdose maybe. I immediately did a 50% water change pur air stones in it. But this morning they still seem the same. What else should I do? Please help I couldn't sleep all night .