Your plant is not a philodendron but rather a Dieffenbachia, and although quite capable of growing indefinitely in water, it is generally not recommended for use in the aquarium because it is extremely poisonous. If a person were to get even a miniscule portion of the sap in their mouth, it causes an incredibly painful swelling and paralysis of the tongue and vocal cords, leaving the consumer mute. If a cat or dog were to nibble on a leaf, they would likely die. My cousin lost her kitten to a dieffenbachia - she woke up to find the plant chewed up and the kitten dead on the floor underneath it. I imagine the plant is also toxic to fish but I am not certain.
If you do intend to keep the plant, it is nevertheless in far too low lighting. I would (ideally) hang a small florescent shop light over the tank, or (probably adequate) position a pole lamp with a CFL light bulb in it directed upon the plant.
Was this plant removed from soil and placed in water? If this is the case then it is shedding leaves because it's soil roots are drowning and rotting and it still is in the process of producing its first waterborne roots, which will be able to support the plant in its new medium. The plant, given proper lighting, should begin to regrow in a few weeks, should you choose to keep it.
But I would definitely recommend a new plant - pothos grows in near darkness and with more light chinese evergreen, spider plant, mondo grass, philodendron, and dracaena would do well.