T.Woods :

I put xmas trees in deep water tied to poles in pond bottom in standing position(top up or down). I use water depths from 17-4 ft. The fish will only go as deep as the theromocline in summer (not enough oxygen below) but from Oct. to May mine use all depths. I also use pallets attached to poles and in other ways and oaks. On oaks I think that the effect on water depends on type of oak (concentration of tannin in sap)and water quality (more alkalinity = less problem) as oak tannin when mixed with acidic water forms tannic acid.