For wood that will have ground contact, use pressure treated available at Home Depot or a lumber yard.  Make sure it says ground contact as there are 2 different levels of treatment.  The wood is pressure treated with a preservative called ACQ or alkaline copper quat. There is another method of treatment which eludes me.  But it's copper base anyway. This is an improvement over the arsenic based materials on the market several years ago.

I would not use this in a fish pond as I believe it is probably toxic to the fish but I would ask someone in your pet store.  Perhaps you can construct a frame of plastic.  You can use PVC pipe (Home Depot) as a support.  ABS pipe is black (plumbing supply) and would be better for appearances, 1.5" diameter pipe with T's or el's to connect would work.  Pretend they're like tinkertoys.