I got a notice from the city that says the total Trihalomethanes are above drinking water standards.

The letter says we do not need an alternative drinking water, but then it goes on to say over years of drinking water containing trihalomethanes in excess it could cause liver, kidney, central nervous system problems and an increased risk of cancer. Lovely.

Wonder what this could do to the fish?

Yea... that's a byproduct of chlorine. That's why cities are switching over to chloramines.
http://www.southerndatastream.com/thm/inde...ml#Introduction

Thanks DataGuru. Very good website. So according to that information it should not be harming the fish since it is removed by active carbon and treating the water.

I think I'd definitel use actiivated carbon in the tank.
Check this article out on carbon
http://www.wernersponds.com/Activated%20Ca...for%20Ponds.htm

Very informative. I'm glad I've been using the carbon ever since I started keeping fish. I have also been using a Brita water filter for drinking and cooking and it has carbon. I will definately keep using them after reading this.

Will sodium thisulphate removes chloramine?

Yes, but it doesn't detox the ammonia released from the chloramine.