You can also use super glue if you want something that works a little quicker. I used to use it all the time for my reef tank when attaching frags to peices of rocks. Its prefectly safe. I dont know if Id use it to glue rocks together but for attaching plastic plants to rocks it would work great.
i've used super glue with no problems just wash with soap after the nice thing is it gets harder in water so it wont dissolve and theres hardly any waiting for it to set