Suction cups are perfectly safe for fish tanks; it's the zip ties you'll need to watch out for. They are fine to be in the tank, but you will want to make sure you cut them as short as possible and that there are no sharp edges. Anything with a sharp edge can cut the body of the fish or rip the fins. The longer the piece of zip tie after you cut, the more of a risk of unintentionally injuring your fish. After you position the zip tie where you want, I'd cut it as close as possible. You can even use a nail file to make sure there is nothing sharp before you put it in the tank.