Make sure you have a lid on it, if you don't either get one or make a cover out of saran wrap and poke holes in it. You can put plants on sides of the tank to stop it from smashing into the glass. Depending on what type of fish it is, get another one of its kind or a few if you have the space. If it is getting stressed you could add a little bit of salt to it if the kind of fish you have likes it.
Hope that helps, Good Luck!