No expert advice from me.

However, I believe all of the old ocean piers I have walked upon on either coast look like they were built with creosote marine pilings. People catch and eat the fish from right under the pilings (but that is open ocean water)!

For most chemicals it is the dose that makes the poison.

I think most of the creosote for old telephone poles was made from coal tar. I don't think that stuff is very soluble in water. I doubt you could get a detectable amount into your fish?

I believe CCA wood is still being sold. (Chromated copper arsenic) So there might be small amounts of copper and arsenic being released into your pond water?

However, I believe most of the algaecides are copper based, AND they can be used in potable water sources. I would think(?) the amount of copper from that treatment type would be more than the copper released from your wood. However, copper can be incorporated into so many different chemical compounds, that I have no clue as to which would bind to something in the water column and become "inactive", and which might stay active?

I know my 2 cents worth above isn't even worth that much money. Just throwing out some ideas that might get you wiser comments below.