When you say open area, do you mean in between the two where the tubes connect them or do you mean there is open space under the tank portions and the bottoms are not fully supported?
The quick answer is acrylic should be fully supported, if not it voids any warranty. Acrylic will flex and most likely sag if not supported completely. Check the bottoms for any deformations
Now if you mean that there is no support for the 5" that the tubes span, then you should be ok as long as everything is perfectly level. Any unevenness between the two sections will put stress on the tubes, so again better to have a stand that completely supports the bottom.
A piece of 1/2" plywood cut to size would solve it.