I purchased a large container of water conditioner and went home to do my weekly water change. Within minutes of my water change my fish started dropping like flies. I looked at the bottle of conditioner and saw that I had accidentally purchased the Balance Plus instead of water conditioner. They were next to each other on the shelf and the labels/bottles were the same color. Who knows what that stuff did to my ph, but I didnt even take the time to check. I did a massive water change and raced all over town to find a place that had water conditioner (pet shops closed at this time of night), near tears. Went home and found more dead fish, did another water change. I lost nearly the entire 35 gallon tank worth of fish because of this mistake, only one Gourami and one Oto survived. Not my only disaster, but probably the worst.
I totally relate to everyone who has said that they felt sick or were in tears dealing with these things. To many people they are "just fish", they just don't get it.