Hmmm. the great water conditioner debate.
Some people say use it, some say don't bother.
Maybe 99% of the time you may get away without using conditioner. And then there is the 1% chance the the council in your area have done a chorine flush, safe to drink but deadly to fish, and all your expensive fish die.
I guess if you choose not to use it your playing russian roulette.
At $3.50 per bottle that treats over a 1000 litres, I'll take the conditioner.
Also I've been told the conditioner contains trace minerals aswell, true or not, I don't know.