What is the cheapest way to stiffen a tube/pipe?
You could truss the tubes as shown in the picture below. The center block helps to support the tube when the truss member is placed in tension. 



The truss could be constructed from heavy wire or threaded steel rod. The concept here would be similar to the scheme used on the wider steps of a wooden step ladder.

I would have suggested the possibility to place the truss wire / rod inside the tube but I suspect that the 18mm diameter of the tube is too small to let the center support in such scheme be large enough to make a significant impact.