That bubble is inside the lines. Which means dang close to level. However, when and where you placed the level to check makes a difference. Things can settle. For example, you can place a stand and then check to see if it is level before the tank goes onto it. Then you put the empty tank on the stand and you check that. You have to assume that the frame on which you plece the level or even the bottom glass of the tank is level itslef. Of course, the weight of the empty tank may cause faults not evident in the stand and/or floor it is on to show up. Finally, you can fill the tank with water, adding most of the weight it will eventually hold and check the level again. More often than not you may see the position of the bubble will change from the start to the finish of a set-up.
This becomes less possible the more solid and well anchored one's set up is. Things can compress with weight, this includes a lot of common floor materials. Here are two things I can tell you. My house is built on a slab and has no basement. Our flooring is wood tiles with an 1/8 inch styrofoam backing is placed directly on top of the concrete . I have tanks from 30 gals. to 150 gals on top of these floors. Some of my stands are bought and sit with only 4 legs on the floor and those legs are placed on oak wood squares to help spread out the psi on the floor. Some are wood stands I built myself and they spread the weight around, but also use 2x4s on edge to support the bottom frame of the tank. I have a 150 and a 125 gal. like this.
Almost none of my tanks on either the stands I built or the typical hobby stands I bought is perfectly level. But they are also not way off. On some tanks one end is likely between 1/8 and 1/4 inch off between the ends depending on how long the tank is. So far nothing has blown out. While the 150 has only been up a few years, I got it used and it is likely 30+ years old. I do have two perfectly level tanks. I paid a finish carpenter to do a custom install by repurposing a large closet. This was about 17 years ago and that leakey 75 was inwall and was replaced. Today that set-up is still perfectly level.
One last observation, the longer any given tank is, the greater a given level error will be whetween the two wnds of the tank. The same degree of error in a 10 gal. and a 180 gal can be invisible when you fill the 10 and noticeably large in a 180. Tanks are designed to be set up level so that the water presses fairly evenly on the ends and then on the front and back, When they start to become unlevel, the pressure increases on the side which is lower. Eventually this can cause a blowout or leak.
