Planted Tank Enthusiast
Join Date: Apr 2004
Location: Des Moines, Iowa
There would be no need to use treated wood in a canopy. Treated wood is no more water resistant than untreated wood, specifically the treatment is for insect resistance.
If it's CCA treated lumber (which is still out there in large quantities despite the phase out) then the words CHROMIUM, COPPER and ARSENIC should be enough to tell you that this is not a good idea for anything hanging over your aquarium.
As for what wood to use, any decent hardwood or hardwood plywood (7 ply or better) that is well sealed with polyurethane would be OK, I've even used white pine with good results. The key is to make sure it's really sealed as any moisture that gets through will cause warping, make joints separate etc. I give hoods three coats inside, and I hit the corners and anyjoints with a heavy coat the first time. It will also help to use moisture proof glue.