I'm going to guess and say it's a 20L.
Need to get rid of the plastic plants. The turtle will eat them, and could very well have trouble passing the plastic though it's system. If some of it get stuck and will not pass, then it will die, or your looking at a very expensive surgery to remove the blockage if you can even find a vet in your area that will see reptiles.
Don't forget, turtles NEED to have a UVB light as well as a heat source for basking. The UVB light provides nutrients they need to grow a healthy shell, as well as a good diet and a basking area in the right temperature. Shell health is of the utmost importance with captive turtles.
That looks like a yellow eared slider. They can get quite large, a 20 gallon won't be suitable for the fella for long. And if I remember right Females grow quite a bit larger than males. For long term, you probably need to think about nothing less than something with the foot print of a 75 gallon (48 long by 18 wide), and maybe even larger than that. Or maybe even better an outside pond. Then all you have to worry about is the filter and the water and some food and let nature take it's course. But the pond would only work out depending on where you lived.
But for now, you really need to get something stable under that tank to support it under all sides. That will cause stress fractures in the glass and cause it to break.
Sliders can be a lot of work to take care of, but they can also be alot of fun and rewarding.
