By "never been a problem" do you mean simply that your fish are still alive, or have you actually checked their gill tissue for signs of damage from chlorine/chloramines?
Why hasn't it happened before? At a guess, I would say you have been lucky in the past. Unless you're testing the water for chlorine every water change you're playing russian roulette with your fish, it may work most of the time, but if something happens and the water treatment plant chucks in extra chlorine/chloramine for some reason you're going to be in trouble.
I think Strollo is right on the money, your low pH is probably what has saved your fish in this case. If it were me, I'd go down to the LFS and buy a big bottle of Seachem Prime (or Safe), and some sort of "bottled bacteria" type product to help re-establish your filter. Drain 50% of the water from your tank, double-dose with Prime for the full volume of the tank, refill and add the bacteria product. Don't feed anything for a few days at least, keep an eye on the parameters and change more water (and use Prime!) if the ammonia starts to creep up again. Big fish like yours will be fine without food for at least a week, so keep the feeding to a minimum until the filters have re-established.
