How many gallons is the tank?
It looks pretty small, only a few gallons maybe? For large tanks leveling is the easiest way to make sure the tank is supported equally on all sides. For a small tank the strength of the tank material (glass or acrylic typically) has a high strength to total tank weight ratio making equal support on all sides is not nearly as critical.
Large tanks are leveled to make sure they are on a planer surface to keep the tank from tweaking and putting undue / unequal stress on the seams which causes the water weight to try to rip the tank apart at the seams. Having the tank base planer is the key. Having the tank level is only for looks.
For your tank I would use long wooden shims under the tank to level the tank if the uneven water line disturbes you.
I would find a way to level the desk because I could not handle my pens rolling around! But the tank itself is probably fine as per
@Oughtsix's post.
Funny enough I don't really notice the desk being out of level while using it. I can see it if I look, but it's close enough that things don't roll. The tank is only 5 gallons, and it was mostly a visual thing. Having worked as a carpenter, and being the son of a land surveyor, things being (noticeably) out of level tend to bother me more than it would most people and I would never be able to relax with a waterline like that!
With that in mind, I made a level-able stand for the tank. I cut down a piece of 1x12 pine to fit the footprint of the tank, then drilled holes and put panhead screws in tee nuts so that it would have feet that I could move in and out. I did this because the round head of the screws will sit right, even at an angle. Then painted it black and put some craft foam on top. When it was time for final setup, I put some blue Loctite on the screws to keep them in place, they were fairly loose in the tee nuts.
It's not perfect but it's way better than before.
If anyone tries this, I'd like to point out a a few mistakes I made:
1) I forgot that the tank has rounded corners. I didn't remember until it was covered in wet paint. So I had to dry it with a hair dryer, then trace the rounded corners on it with a pencil (hard to see over black paint) and then had to go back outside and round off the corners and repaint them.
2) I got short screws, which wasn't a problem except for in the lowest corner. It's still a bit out of level but I've gone as far as I feel comfortable going on that short screw. Get a variety of lengths if your surface is severely out of level. I used 1/2" long screws, I should have gotten some 3/4" too just to be safe.
3) The screws scratched the desk. It's an oak desk finished with Danish oil so it's not as noticeable and pretty easy to touch up, but be mindful of how much you slide it around. Little bits of felt under the screw heads would be a good idea. Or a mat under the whole thing.
