Looks like a lotus/lily bulb to me. It'll prolly do best with 2.5-3WPG and a nutrient-rich substrate like Flourish. I have three of these growing near each other, and I'm aggressively pruning/training them to stay short (6-10" tall). Under the right circumstances, they can be very very pretty and have wonderful flowers.
Also, make sure you haven't completely buried the bulb. Leave about half of the bulb sticking out into the water. This pretty much goes for all aquatic bulb plants. If you don't have a rich substrate, you can do like me and bury a Flourish tab next to the lily, as far down as your finger can push it.