Practically all tanks should have some live plants, not including the algae growth that will occur. Algae is a lower form of water plant without flowers and a simple structure. Algae blooms are usually caused by excessive amounts nitrate, phosphate, too much light, or exposure to sunlight.
Live plant have many benefits including:
Plants absorb nitrate from the water because they use it as fertilizer.
They help reduce the amount of algae by using the same food sources it does.
Plants take in carbon dioxide and produce oxygen (during the day with adequate light). At night, or when the light is not enough, they use oxygen and produce carbon dioxide.
They also gently flow in water current giving the aquarium a very natural look.
Aquatic Plant Needs
There are basic needs that aquatic plants have starting with being firmly planted. Bury the roots in the gravel after cutting off any unhealthy roots. Then place the plant deep in the hole, fill in the hole, and pull the plant up a bit. Don't bury leaves, if there are some near the bottom cut them off. After they're planted you may need to increase or decrease the following factors as you watch the weekly progress of the plants. With all of these factors interacting it could be months before you reach a state of equilibrium.
Fluorescent Light: Get the type of bulb that is designed to stimulate aquatic plant growth. Then use it for 10 hours per day, setting up a timer would be ideal. Intensity depends on the plant and how deep it is in the water. Here are popular starting point guidelines based on light requirements:
Low Light: 1.5 to 2 watts/gallon of water
Moderate Light: 2 to 3 watts/gallon of water
Bright Light: 3 watts/gallon of water
Carbon Dioxide: This is vital and if the amount produced by the fish is not enough then it may be added through carbon dioxide injection equipment into the water, especially if there are many plants. Low surface agitation should be maintained to lessen the oxygen [O2] and carbon dioxide [CO2] exchange, the plants will use the CO2 and produce the O2.
Fertilizer: Plants use nitrate as a fertilizer, but plant food may be introduced by using slow release fertilizers placed on the bottom. It's best to feed them little and often rather than a lot in a great while. Refer to the plant food manufacturer's recommendations if you use any.
Heat: In the ranges of 70°F to 86°F for most species.
How Many Plants
If you want to keep a few plants to help absorb the nitrate, reduce the amount algae, and make the aquarium look better. You can arrive at your ideal plant amount by first measuring the nitrates and scraping off the algae, then in a week take another nitrate reading and clean off the algae again. This will give you a feel for how fast the algae and nitrate grow. Second, add some plants and in a week see if the algae growth was less than the week without the plants. Then scrape it off again and add more plants, after a week take another measurement. You can keep doing this until the algae growth is very slow or not recognizable and the nitrate readings are about 10 ppm.
When to Fertilize
Here are some general guidelines for when to add plant fertilizer to your tank.
Small amount of fish, many fast growing plants, and strong lighting (3+ watts/gallon): Needs full amounts of fertilizer, and carbon dioxide injections.
Average amount of fish, fast growing plants, average lighting (2+ watts/gallon): Needs small amounts of additional fertilizer.
Large amount of fish, few or slow growing plants, and low lighting (1+ watt/gallon): No additional fertilizer needed because fish waste should be sufficient.