Yep - stress coat will almost immediately nuetralize cholorine.
Based on your response I'd be very wary of the
PH factor.
PH, especially large shifts can stress fish quickly and can even kill them in a few hours. There is no question about that.
What can happen is that if your tank is established it may have been slightly acidic - (due to the type and size of fish this is a likelihood) but your tap may be alkaline. Let's assume that your tank was running at 6.5 (good for the fish you have) and your tap is running at 8.5. Each single digit change in
PH is 10 times more acidic or alkaline depending on the direction of the change. So in your case from 6.5 to 8.5 that is a huge change and if you changed 50% of the water this may have caused a jump from 6.5 to as much as 8 or even right up to 8.5 temporarily depending on the buffering capacity of your water.
I'd recommend you get a
PH test kit (invest in a good one and macke sure it is suitable for fresh not saltwater and test both your tap and your tank).
The oppisite could have happened as well, i.e. your tank had a high
PH, the fish got used to it and your tap has a very low
PH and your water change caused a "
PH crash", used up all of the buffering and the
PH dropped quickly and drastically. In this case ammonia poisoning would not likely be part of the equation.
Most fish will tolerate a large variety of
ph levels but they do not like fast changes.
Also recall that Ammonia is much more toxic at higher
PH levels (this is because the free hydorxyl Ions combine with the less toxic Ammonia producing more toxic Ammonium). This is why Ammonia levels are considerably more critical in
SW tanks which run at higher
PH levels.
Anyway
HTH.
Tom
