Aquatic plants do best with 2 watts per gallon fluorescent lighting (or metal halide) with a color temperature of 5500-8000 degrees Kelvin (color temperature). From all articles I've read on aquatic plant lighting and lab results 6700K is optimum, it is just slightly more intense then the suns light at 12 noon. Lighting from an aquarium store can get costly, especialy M.H. You could save a bit of money getting 4'shop lights from Home Depot or a discount store. Get the 48" bulbs/tubes from Home Depot, I have found them to be the cheapest. Get The "Daylight Deluxe" by Phillips. They are 6500K and only cost about $5 for a 2-pack. 3 fixtures (double bulbs) and 6 bulbs should be fine. The 48" tubes come in 2 wattages 32W 0r 40W. Either one is fine and will bring you close to the 2W per Gallon. Put them as close to the top of the tank as possible. The aquarium should have a glass canopy. If not while at the Depot, get some plexi-glass/acrylic or even less costly in the ceiling tile dept. are clear panels for drop ceiling lighting. Cut to fit. I do not discourage local pet stores, but even DIY gets a little costly. If you have the cash get the fixtures from the pet store. Compact flourescents are more efficient than the shop lights, and M.H. is even more efficient. The glass canopy is also available at the pet store.

I wouldn't bother trying to keep plants like hornwort going. They're fairly inexpensive and easy to replace. In _just_ 100 gallons, you don't want the plants to start rotting/disintegrating clogging up the filter. It's different in a pond setting where the filter is probably larger/stronger. Not sure if you have a skimmer set up in your pond outside.

You don't need a complete light set up. If you really want some plants in with your stock tank, concentrate the plants in one area and set up a stronger light source.

This is true you can even use those portable spotlight/worklights that have a clamp and cone shaped reflector and a compact flourescent daylight bulb and direct it at the plant. Then again to have full lighting with the shoplight method would only cost