Trihalmethanes are a byproduct of organic materials reaction to chlorine. If the incoming water to be treated has a high organic load, more trihalomethanes are created, so high concentrations may be seasonal.
They are known to cause cancer in lab animals at high concentrations, so most research has been done on how they effect humans, when I was in water treatment school little to no mention was made on the effects on fish.
Most water suppliers are switching from straight chlorine to chloramine to avoid the production of trihalomethanes in drinking water.
For fish keepers, it is a double edged sword, because straight chlorine treated water, is easier to dechlorinate, simply by letting it stand, because chlorine dissipates quickly.
Whereas chloramine maintains its residual for long periods, so though safer for human consumption, a bit harder for the fish keeper to deal with.
