if i wanted to do that i would buy some 50x25x2.5 Aluminium rectangular hollow section (RHS), put 4 rows on there edge on top of the stand at the same length of the tank & then fit a layer of ply for the poly & tank to sit on. I would then get timber or ply & fit to front & end of the new platform to tidy it up.
I don't think it will look right but that may not be that important.
Ali is generally cheap in comparison to other materials.
Ullrich RHS
http://ullrich.co.nz/rectangular_tube1.php