Unlevel tank worth keeping?

[Seisage]

So I have an opportunity to grab an unused 25gal tank for free from one of our storage closets, which would be fantastic. The only problem is that when I filled it up to do a quick and dirty leak test before taking it home, I noticed that the tank is sitting noticeably uneven with the water level. I'm very confused, considering I've checked and re-checked along all of the tank edges and the ground it's sitting on with my phone's level. I even checked to make sure the plastic rims were fully on the tank, so I know it's not just an illusion. Everything is perfect, but the water level still looks wonky. The only thing I can think of is that the bottom pane of glass itself might be sitting uneven, but until I drain the tank, I won't know for sure.

Is this something fixable? The difference between either end of the tank is 12mm (just under 1/2 inch). Even if it's not dangerous to the tank's structural integrity, it is somewhat visually distracting... Is there any way this can be remedied, or should I just go for a different—although sadly not free—tank?

Please don't mind how disgusting it looks I filled it using our seawater system, and since it comes straight from the bay and this particular plumbing is pretty old, the water can and often does look kinda gross.

(Edited title. No questions as to whether this tank is unlevel, I think)

 

[TNG]

I think the floor is sloping to one side. You can check by filling a glass with water and place it on the floor. If it looks like this then it's the floor. It's not uncommon for basement floors to slope to one side for drainage purpose.

 

[Seisage]

I think the floor is sloping to one side. You can check by filling a glass with water and place it on the floor. If it looks like this then it's the floor. It's not uncommon for basement floors to slope to one side for drainage purpose.

I can try checking with the glass of water in the morning, but I definitely checked the concrete floor with the level and it was a perfect 0 degrees. I do realize that the photo I took makes it look like the floor is sloped, my bad. The way drainage works in this particular area is through plumbing that drains directly into built-in troughs, so there shouldn't be a need for the floor to slope. I could be wrong though, I suppose. It would be much nicer if it was just the floor and not the tank.

 

[Seisage]

So it was the floor
Even though it's after 11pm, I went out to try the glass of water. I couldn't see any effect on the water in the glass, but I just had a hunch, so I took the tank back to my place to set it up in my bathroom, where I know for a fact I have a level floor. And yeah, it's totally fine... I feel a bit silly, but I guess that tells me to not trust my phone's level function! And hey, I'd much rather feel a bit silly than give up on a nice, free tank.

 

[GaryE]

I have an old fashioned 3 foot long metal level, it is has saved me from a few optical illusions, and from measurements with shorter levels.

Some tanks can change form after a long time uneven. I have one. They do tend to fail, so your concern was justified.

 

[Uberhoust]

Check the plastic molding along the top of the tank. These sometime get loose and can pull partially off giving the impression of it being miss leveled. To check the actual level, with a level (pun intended), just measure the depth on one end and compare it to the depth of the other, simple tape measure would do the job.