I've just downloaded this small software and it does wonders!
Of course, I've been using to design my dream tank! 
You can find the software here (it's free): http://sketchup.google.com/download/
Here are some pictures of my first trial: It's a 3 x 1.2 x 1.2 metres tank, the stand (crudely designed) is 90 cm high and there are two overflow boxes (plumbing and tank bracing not included) on the sides.




You can find the software here (it's free): http://sketchup.google.com/download/
Here are some pictures of my first trial: It's a 3 x 1.2 x 1.2 metres tank, the stand (crudely designed) is 90 cm high and there are two overflow boxes (plumbing and tank bracing not included) on the sides.





