Plants do several things for fish-
1- Absorb ammonia (preferred) and nitrates (sometimes)- can virtually eliminate the nitrogen cycle, keeping nitrogenous compounds in check.
2. Convert CO2 to Oxygen, helping the fish.
3. Harbor infusoria for young fish to treat as a first food source (If egg-layers happen to spawn)
4. Provide shelter and comfort to fish and fry.
5. Plants can help
remove toxins from the air that end up in the water, or trace elements that are in tap water.
If you search the forum (on the top of the page, between new posts and quick links, or just click here
) for "Low light" plants, there will be several topics.
My own reccomendation? Buy a good liquid fertiliser when you can, and make sure you have proper lighting. (It's cheap, don't worry.)
For lighting, use flourescent tubes (Or if you have a screw-in fixture, buy some CFLs AKA Compact Flourescents- the spiral bulbs) and make sure they say "6500k". They are often sold as "Daylight" or "Cool White", but make sure you see 6500k (6700k is ok too). Don't worry about the wattage, and just get what fits your hood.
For fertilisers, grab some "Flourish Comprehensive", and dose it once a week as directed.
You do this, and you can grow nearly anything. If you can't afford new lighting, there are a couple plants that are nearly bullet-proof, but they appreciate good lighting and ferts too.
Echinodorus species AKA sword plants(many get large)
Anubias, Java Fern, and Java moss grow slowly and should be tied to decor or gently placed on the substrate (not planted). They all grow fairly slowly, but can get fairly tall.
Stem plants (most of the others I named, often called bunch plants) have no predetermined height- they will grow forever. When they get too tall, just cut them to the height you want. You can even get the parts you cut off, remove the lower set of leaves, and plant them to form new plants.
Dwarf sag will grow taller in low light (4-6 inches), but will stay short (<2 inches) if it's well lit.