I found this info on the net:
The old style (CCA) treated lumber is slowly being phased out, but will probably be available until 2010. The new style lumber treated with copper sulfate is not much better as far as fish compatibility goes, as even small amounts of Copper leaching out of the treated wood, can kill fish!
The reason for not wanting pressure treated CCA wood over a pond is the Arsenic and/or Copper in the pressure treated wood, can build up in the water by the leaching effects of rain or even heavy dew and kill the fish and probably the bacteria in the filter. In addition, some of the chemicals from the pressure treated wood may stabilize ammonia so it cannot be converted by the bacteria, or the bacteria may die off so fast that no biological treatment may be possible with the chemicals from pressure treated lumber present.
Safety First
To be safe, neither the CCA or the Copper Sulfate treated wood should be used over a pond and certainly never
in submerged applications. Both kinds should be painted with a sealant, if there is any chance the water will come in contact with the wood, this however will weather off yearly and must be redone..
Other construction materials to consider:
Man made resin composites
Redwood
Cedar
Cypress