Nice start... I would prob go with some sort of fixed base around the bottom of the legs in order to help prevent bowing out. Especially with the weight of the water in the tank. Also remember that the 18 gauge aluminum braces you have on the tank legs, unless attached from both sides will definately affect the stability of the whole system. Please keep in mind to add a styrofoam layer under the tank in order to accommadate any little variences in height from uneven boards. But I'm sure that you arent done with the stand.
Don't mind me.