right here is a link to what i am talking about. there 3 main kind of bracing, compression, sheer and moment. compression is up and down which your 2x6 are more than enough. moment is a twisting motion which you have but not much. sheer is a left to right movement which is what im talking about. if you put weight on that stand as it sits now, the weight of the tank will tilt it left or right and that whole structure will crumble
http://www.structuremag.org/images/0807-sd-6.gif