I would like to get some pond plants for my fish tank.?

This is a very cool idea which I wish more people would try. Most pond plants do best with emergent growth, so your setup is perfect. Lilies will produce some lovely reddish submergent leaves as well as a few floating lily pads, and Goldfsih tend to leave them alone. If you like a swampy look, mangroves and bamboo could be nice touches around the back, and would lend themselves very well to a taller appearance. Arrowhead, Lizard Tail, Cardinal Flower, Cattails, and most of the plants found in Home Depot's pond/water garden section wold work extremely well and produce flowers in some way, shape, or form.

For substrate, their are planted pond substrates that can be bought very cheaply and be very effective. Home Depot sells a good pond soil which you could cover up with WELL RINSED play, pool filter, or blasting sand... http://www.schultz.com/ProductCategories/Soilsamen...

try the banana lily!!! this would work perfectly for you!! you wont need substrate for any plants. the goldfish will produce waste which eventually produces nitrates, which is basically fertilizer for the plants. with goldfish, adding substrate will produce too much excess minerals in the tank for both the fish and plants.

caution, snails sometimes lay their eggs on plants so if you pull them up from a pond they may introduce snails to the tank. I quarantine them for a couple of weeks and if no snails develop then I add them the plants to my planted tank