just did a water change on my 72g and i am not sure what went wrong. i had 3 zebra danio in it and 1 died about a half hour later and the other one was gasping for air at the top and is now freaking out and not swimming right. this one will most likely die sadly. the third one is doing completely fine. checked everything after and ammonia was 0 nitrite was 0 and nitrate was i think 20. i matched the temperature perfectly and added water conditioner about a gallon or 2 into the fill. i used these fish to cycle the tank and they were completely fine throughout it. i would say i did a 25% water change. what could i of possibly done that killed 2 fish in thirty minutes and an hour. this was the first time i was adding water right from the shower head and didnt have to squirt the conditioner into buckets. could i of added too much conditioner? i was hopin this time i would not lose any fish, even ones i used for the cycle. thanks in advance