I would expect the main problem is the possible leaching of any chemicals from the wooden decking. Any wood preservatives, colourings or coatings used must be fish safe. If the decking gets wet or is open to rain then you don't want any nasty chemicals getting into the pond.
Perhaps stonework with say a coating of clear G4 is a safer option.
Merry Xmas
Paul
Posted on Dec 21, 2006, 9:17 AM from IP address 81.137.222.103