When you create a wooden holding tank or aquarium, the method of construction that you use is going to be very important. Most people forget just how much water weighs and this fact alone can sink a wood tank.
Make sure to use horizontal ribs, not vertical ribs. Horizontal ribs will provide a structure that allows less bowing of the side of the tank than any other method I know. Also make sure to plan to have a cap of some sort. this will lock the top of the walls in place and keep them from bowing as well. Make sure that the bottom is ribbed too.
You might think that having these horizontal ribs is going to make the wooden tank look bad aesthetically. Remember that this is just structure. You can always cover them up with a very nice looking wood after the tank is built and sealed with Pond Shield. There are nice looking hardwoods that can be polished into some real nice looking furniture quality pieces.