Philodendron isn't the best choice if you want to emerse land plants in your aquarium. Pothos, a variety of Philodendron works much better and even better is Alglaonema (Chinese Evergreen). Anyway, check the plant you have to make sure all the potting mixture is removed. You need to aerate the root ball. I use an air pump and run a line of tubing under the water and directly under the plant roots. This will provide the roots with plenty of oxygen and move the water with the dissolved nutrients through the roots. Clip off the yellowing leaves, if the plant isn't too far gone, this will stimulate new growth.
Philodendren does best with a light source in addition to ambient (room) light. This time of year the light in the room is dimmer. So, get an inexpensive pole lamp and put in one of the higher watt eco bulbs and keep the lamp on at least 12 hours per day. I have mine on timers.
Just a couple of thoughts.
I agree agalanomeas or chinese evergreens are best plants. Does best In hydroculture. :3