Water Change Killed Fish.

Question
Hi Nick, I have been running a Red Sea Max Aquarium with fresh water fish in
it for about a year now. Every week I changed about 15% of the water,
cleaned the gravel and cleaned the glass all with no problems. My tank was
thriving and all of the fish were very healthy. Today as soon as I had replaced
the water with fresh water I noticed the fish started to behave oddley,  within
about 15 mins all of my fish were dead. I am so annoyed and baffled, do you
have any idea on what might of happend ?
The tank is 34 gallon / 130 Litre.
The fish were, 1x Pleco, 5x Tiger Barb, 5x Serpea Tetra, 1x Albino Rainbow
Shark, 4x Zebra Danio
I always use Tetra water Aquasafe and Easy Balance with each water change.
The Filtration system is a built in machanical and carbon system .

Answer
Well, for starters you can get rid of the Easy Balance, no point in that.

I'm guessing it was a temperature change. You must make sure the temperature of your water is exactly that of your tank.

Also, 25% twice a week is the minimum for next time.