Waterproof wood "sealer" for resevoir
I built a couple bogs/planter boxes for filtration of my small backyard pond using some MCA treated lumber from local box store. Water will be pumped into them, filtered up through the pea gravel/plants, and then flow back out through the waterfall. I can silicone all the joints to make it watertight, but what can I use to seal the wood itself that won't break the bank and isn't toxic to fish? I know epoxy or liquid EPDM would work, but those are expensive. I also want to avoid using a pond liner or plastic liners, as I need the bulkhead flanges on the outside.
Would just simple polyurethane work? My understanding is that it's plant/fish safe once fully dried.