Our backgrounds no longer come with clips, however you can now buy them (more details here) to secure them in place.
Some backgrounds won’t work with the clips because they are too thick, similarly some tanks won’t be able to use them either because the glass is too thick or something is in the way.
In those cases silicone can be used to hold it in place. Generally the background should be tight enough that the gravel at the bottom and tension from the sides holds it upright. Also in many cases a decoration or 2 leaning on the background can help hold the background up.