Angel, if you look at engineering tables you will see that one 2x4 column alone supports over 600lb in compressive load. I remember you built yours from larger sections and I don't oppose using materials one already has on hand (I build a lot of stuff from salvaged wood as well), but in reality there is no need for that whatsoever. Four 2x4s in corners will already support several times the load OP's aquarium will impose, and we usually use 4 or 8.
As Tom mentioned no center support is really required. The only reason I have a center column under my 125g is for placement of doors.
ADD:
Single 4x4 column of southern pine will support over 7000 lbs. I'd say your stand is overbuilt by orders of magnitude, will support a story of stacked 75gal tanks if not more, lol