At first I thought there might have been a problem also but then I looked closer. If you had not added the second upright 2X4 under the ring, the weight of the tank resting on the ring at the top would have been supported only by the strength of the screws. Dry wall screw are not made of as good metal and not as thick as deck screws. If they had been the sole support, they might snap and let the whole thing down. But with the second 2X, the weight is on the upright wood rather than the screws. Black heads are the giveaway on type?
For the surface, I would not sweat the little things if the stand is reaonably true at the corners. Tanks are like box beams with really strong but brittle sides. The sides may crack if they are twisted by one corner being high or low. But as long as it is not twisted, it is nearly impossible for a tank side glass to droop when it is standing on edge. Like a 1X6, on it's flat side you would break a 8 foot board just walking on it but laid upright on edge, you can't bend it at all. If you have the four tank corners level, the middle is not going to droop.