The care and feeding of Aquatic Plants
The most important thing to know about plants is their light requirements. Beginners should only buy "low light" plants and leave the rest for the better equipped. You'll have to buy a "full spectrum" bulb, and keep it on about twelve hours a day (use a timer); many of these plants are native to tropical areas where the days are twelve hours long. They will also need fertilizer- especially iron. Only use fertilizer made especially for aquatic plants. I'm using "Kent Freshwater Plant" fertilizer.
Plants also need carbon dioxide as a fertilizer, and gadgets are sold for this purpose. If you want to, you can "do it yourself" . Bubble it in with an air stone attached to an airline whose other end is inserted into a hole in the cap of a beverage container - you fill it about two thirds of the way with warm water, add a teaspoon of yeast and a cup of sugar; the yeast will make the carbon dioxide which will travel up the tube and into the tank via your air stone. When it stops working, add more sugar.
Don't worry about the gravel, It's main purpose is to hold the plant down. Most true aquatic plants get much of their nutrients directly from the water through the leaves. Just press fertilizer tablets into the gravel around the base of the plants. Just as you do for your fish, research the individual species before you buy. Don't buy plants that don't belong underwater. Believe it or not, some pet stores will sell you a terrestrial plant, hoping it will live in your aquarium (one common culprit is the spider plant).
Beginners with small tanks should stick to hardy plants that do well in low light. Your tank might look bright to you, but a lot of that light does not reach the plants because light does not travel well through water. When choosing plants, keep in mind how tall they will get and how high your tank is. Vallisinaria are a good choice for new tanks because they grow and multiply quickly. Just be careful to get the shorter ones. I prefer Saggitaria. I like dwarf Saggitaria, Anubias, Java Fern (Microsorium pteropus), Ludwigia repens and the Cryptocorines (I like the reddish C. wendtii). If you want to buy only one plant to toss onto the surface to float (and grow to cover the surface), get Egeria densa (anacharis).
Live plants do have a purpose in life. Plants add oxygen to the water, and absorb some of the nitrates produced by the nitrogen cycle. They also provide food and shelter for the fish, and my Rosy Barbs are very fond of depositing their eggs at the base of the plants. Other fish will use the leaves for this purpose.
Stem plants are often sold in groups with a metal band wrapped around the base. You need to remove that band. Hold the plant under water while using a SHARP scissors to cut it off - cut well above the band, where the stems are green and healthy (the band often crushes the stems). Don't be afraid to cut high - cut it where it's certain to be healthy, and it will sprout new roots at the buds. (Dead roots are brown, healthy ones are white). It's best to hold it under water when cutting, because it avoids getting air bubbles into the stems. When planting stem plants, don't plant them in a bunch as they were sold. Separate the stems and plant them in separate locations. They will grow in - and when they get tall, you can prune off the tops and plant those too.
As far as planting depth and water conditions go, that varies by species. Consult a good book (I like "Aquarium Plants Manual" by Ines Scheurmann). You can also visit my LINKS page. If your plants look sick, they probably need fertilizer.
Algae is not a problem for the fish. Some of them, like Livebearers, enjoy eating it as a snack. It is a problem for the plants if it grows on the leaves and prevents light from getting to the plant. It also may cover the hood, and prevent the lamp's light from getting into the tank. You should wipe this off every time you clean your tank. To prevent algae, Keep your tank away from direct sunlight, have enough plants to compete with it for nutrients, and change the water on schedule. Include some algae eating animals in your tank, like Otocinclus affinis or, if you have a large tank, a Plecostomus. All aquarists get some algae. I scrub my glass with a Mag-Float; it is much easier to use than the traditional algae scrapers. The operative word here is "float." If you drop the magnet into the tank it will float on the surface until you retrieve it. Don't get a brand that sinks. I do not use algae removing chemicals - it is unnecessary and toxic. There are links on my LINKS page that will help you identify what type of algae you have and what you can do about it.
FILTERS AND STUFF
THE NITROGEN CYCLE
DISEASES AND MEDICATIONS
PLANTS AND ALGAE