Any wood can be used in a pond. Just submerge the piece of wood in water for a few weeks before putting it into the pond so it will become water logged and sink and so any toxins leach out of the wood.
I’ve collected a lot of local wood and this is what I’ve found…
Wood such as root balls that have recently been excavated almost always grow mold / white fungus when put into a tank/pond…
Wood that has sat exposed to sun / weather grows mold / white fungus about 50% of the time and often begins to break down once in the water…
Wood collected from river banks or other shore lines probably didn’t spend as much time in the water as you would think. This wood works out more often than the above sentence’s wood, but still has a high ‘risk’.
Wood that is submerged in a body of water is in the same condition in nature that it will be in your tank/pond… it’s much harder to collect but this is the wood you are looking for. Best to get it right from the start in my opinion…
A friend of mine is a City inspector and he is overseeing a redesign of a local pond. I’ve visited him and collected several large pieces… all seem to be great finds… An opportunity such as this may keep you dry in the collecting process, but dragging logs out of a pond isn’t too rough… a rope tied to your bumper hitch may makes things go a lot easier (the hitch, not the bumper, you’ll be amazed at how well suction will hold a log in place…