That is absolutely crazy! i cant believe that. i agree with the others that the water changes were way too much though. i am even scared to be doing the 50% per week that is suggested with my planted 75. i have a redtail barracuda in there and they are way to picky to accept that kind of a water change. i do only 40% per week and that is still high. if there was some kind of chemical in your water, there is no way that the fish could have survived it with that much of a change.
i am amazed that it was that fast though. what were you using to change the water though! i used a bucket once that my fiance thought was just a bucket and put cleaning supplies in once. i have special buckets just for fish water only. she took that bucket and cleaned the floors with it, then i changed my water and walla, dead fish.
it must have been some kind of crazy contaminant. probably not in your water because you are still fine after drinking it. however it cant be just coincidental that it happened after a change in product.
i dont know man. that is just horrible and i would cry if that happened to me. my cudas are dope and if they die, i dont know what ill do.
i hope for everyone your sake and everyone else who is reading this, you figure it out. please post the reason if you get it figured out man.