After all the info and help with this i am going with all wood, mite aswell as there arnt enough benefits to make a tank from steel.
Earthstudent,
My first plan was to build it from wood with a similar design to the post you gave, My design involves 4x2 and 4x1 timber with 3/4 ply and a few layers of grp. To make it stronger i will be doing moyice and tenon joints on the uprights going into the top and bottom frames.
I mite start a build thread soon after xmas once i have got a bit of the garden done and the build area as i will be building it on the other side of the garden them lifting it into place.
thanks again all
wayne