idk about you but a 75% change is a little much. unless you need it it seems like overkill to me. i like to do 25% to avoid things like this. just incase the water/conditioner is bad. i started doing this after i killed everything in my 220 gallon (redtail22", tsn 22" and about 10 other fish) and havent had a problem since.
i also have a tank with some really scetchy fish like neons and angels that i do a water change on first. if they die i know something bad might happen to my big tank.
but in your case they dies 5 min after, i have forgoten not to add declor for hours and they usually dont die. maybe your water temp was hot coming outa your hose/siink? or like the guy above said maybe you had bad water coming in your house. just so quick doesnt seem like the declor was bad.
sorry about the fish, it is the same feeling as loosing a dog or another pet, just worse because you feel like you did it to them......
i also have a tank with some really scetchy fish like neons and angels that i do a water change on first. if they die i know something bad might happen to my big tank.
but in your case they dies 5 min after, i have forgoten not to add declor for hours and they usually dont die. maybe your water temp was hot coming outa your hose/siink? or like the guy above said maybe you had bad water coming in your house. just so quick doesnt seem like the declor was bad.
sorry about the fish, it is the same feeling as loosing a dog or another pet, just worse because you feel like you did it to them......