That’s a pity really.
I can relate to being annoyed at something that is just there and being a perfectionist I know it would just drive me a bit nuts.
When I first started, I did use background posters but only used tape to stick the posters on by the edges of the tank, then graduated to black cardboard and stuck those to tank. Until finally settling on actually painting the back of glass with black paint on my last few tanks I set up.
Painting back of tanks is the way to go for me as this ensure it’s a uniform and perfect background in my opinion.
Cheap acyrillic black paint is perfectly fine to use and use a small roller to paint this on the glass evenly, that does the trick nicely, takes two or maybe 3 coats to cover 100% of the back glass.
Of course this would mean emptying this tank and setting up to paint etc, but for future that may be something to consider next time you set up a new tank.
But for now, maybe chalk it up to learning experience I guess and maybe try a black card for this tank, if you can get the glue off the glass etc. Trail & error.