The plant is a cordyline, you can still keep it just pot it up into some compost and keep it in a warm bright place near to your tank it should grow quite happily for you in this way, it's because it likes moist soil that there is a belief it will survive submerged but it is not a true aquatic therefore it won't, it will be fine for a short time and then it will die, as someone has said if it is rigid when it is removed from water it is more than likely a terrestrial plant, true aquatics go limp when taken out of their home.