As celticwrath mentioned, you need to match the plants to the light and to a lesser extent the substraight. The lights that come with most tank setups are not suficient to do much more than showcase your fish. The lower light requiring plants such as Java moss, Java fern, Anubis and I think Anacharis (ask at an aquarium store; the petstores that have fish may not know much) need at least say 2-4 two foot flouescent 40 watt bulbs (the shoplight kind) for a 20 gallon tank to grow. That's two or four bulbs lying across the top of the tank longways. The deeper and bigger the tank the more light needed. If you are growing grasses and plants demanding more light you need to invest in a higher output aquarium light(s) which costs a lot more $. Have the lights in either case on at between 8 and 10 hours a day to simulate the natural night/day cycle, which you can either accomplish with a timer, or by you keeping track of a schedule. If you go ballistic and have a CO2 injection system you will probably need fertilizer for aquariums as well, but in most cases I don't think you need that. With fertiliser you can have problems with algae. Don't use potting soil for a number of reasons. Most plants in the lower light catagory which I suggest you use because they're much easier to deal with don't need any particular substraight to grow in my experience. Just anything sold at an aquarium store for growing plants in should be fine. I've used sand from the beach (rinsed!) and grew lots of plants, had f/water clams, etc, but I wouldn't advise anyone to use that! . Make sure you don't over stock fish in the tank or feed too much so the biological cycle through the filter(s) can function correctly and feed your plants rather than kill them and you should have a beautiful flouishing fishtank full of gorgious greenery.