the big water change most probably caused this.
sometimes the source water is not ideal. sometimes the tank water is much better. or should i say the fish is used to it.
even if it seems old or needs changing the fish may have slowly adapted to it and may not be getting sick. this is often why fish that have been in a tank for a while will live, while a new addition can die seemingly without the tank water itself being the blame.
for example new koi into low pH water often results in this phenomenom while the old ones survive.
sometimes the number of times a fish is handled or moved from water types to another adds up to the death.
i knew a fish farmer that said with his water. if he changed it three times in a row over a few days the fish would start to die.
this was from a creek. and with goldfish!
where did the source water come from? what temperature was it? is it possible that the tank water had a much higher element profile than the source water? therby maybe the source water was like rain and too pure? and resulted in a drastic change.
it could be a number of things adding up too.
my guess if you were to check the tank water prior to doing a major change and then check the water you wanted to use to exchange. that there would have been a big difference somewhere there. dont forget carbonate levels etc..this is why really, large tank volume exchanges should be avoided. UNLESS it has been shown that the species really can handle your source water under a flow though kind of set up.
now to further add drama. say you have your fish in the tank. and you start a flow throw and ran it for days.... and the fish dies the first night. well, it is possible that if you were to flow some water in and do it slowly that the fish can then tolerate days of flow through. i am not suggesting flow through systems at all. what i am trying to say is that shock can occur fast enough to kill while other times if its gradual the fish tolerates it.
