Idk how much actually will each into the pond if any once the PT lumber drys out. Both of my ponds have PT lumber around them, the main 10000 gal ponds case its In the pond in the form of 6x6’s holding up a bridge. Been like that for 11 years now with no issues.... The other 1800 gal. pond is half raised out of the ground framed with 6x6’s that contact the water when it is full and run off it when it rains, this pond is going on 3-4 years old without issues. Just my experience, with it.
@poconojoe you are correct the PT lumber from the big box stores has copper in it still ( no arsenic though)along with some other stuff that is extremely corrosive to plain metal coated screws( the corrode away within a few years). This didn’t happen with the cca treated lumber.
All the lumber I used is still CCA (PT) lumber, CCA stands for chromated copper arsenic, this is what is on my pond. Except for 2”x material as no one can get that in cca anymore, unless it is govt work then it’s fine.
If you let the PT lumber dry out it should be fine, clean off all the residue. IDK about stains that are fish safe, but I wouldn’t trust any type of chemical liquid stain or sealer around a pond to be fish safe. Even if you stain it first and let it dry then build the bog, in a few years or less you will need to stain/seal it again, which will be impossible without spilling it into the pond. Also, big box store lumber is junk even the PT lumber if it is left exposed to the sun or gets wet consistently, touches the ground... I would build the entire bog Framing out of 4x or 6x material, so it will last as long as possible.