Answer

To keep Elodea pond weed for longer periods after purchasing, place into an aquarium or similar tank and anchor with gravel or soil - or tie a small bunch together near the bases and attach a small weight; the weed should then begin to grow.
It grows best in a nutrient-rich, high light environment. Small amounts of plant feed can be added to improve the nutrient content of the water. For lighting, either keep the tank near a window which receives some sunlight or illuminate with full spectrum (5000-7000 K) LED or fluorescent bulbs or LED plant grow light panels etc.
The preferred pH is 6-8, although a wider range can be tolerated. A temperature range of 10-26C is best, although lower temperatures can also be tolerated. Soft water is preferred to hard water.