What style tank is it?
If it is only held up by the perimeter, that 'beam' wouldn't be holding any weight and you could make a simple cosmetic fix...
If the entire bottom rests on the stand, then it would be a 'structural beam', although the weight could (most likely) still be carried by the paremter only. Provided this is true for your tank, then you could "get away" with a cosmetic fix, but may prefer a structural fix...
For a cosmetic fix I would put some glue above it, press it back up in place brace it until the glue dries...
For a structural fic, I would do the same as above, push a couple screws into it and paint over the screws...
Also, as mentioned, I would consider putting a screw or two in all of the others as well...