Lotus pods are buoyant in water, so you'll want to weigh it down before placing it in your aquarium. Gluing a rock to its base with non-toxic waterproof glue will do the trick. If you have a pod ...

As an added bonus, lotus flower seed pods have tons of uses ... fluffy potting soil will float away when you add water). Dig a shallow trench in the soil's surface and lay the tuber in it.