Algae
Algae is at least one time or another a problem, weather it is big or small. There are many types of algae. Here are some you may encounter:
Brown Slime/Film. This is the most common for new fish keepers. It starts out as rusty colored specks then grows into a brownish slim/film. A Pleco can take care of this easily, possibly clear it in less than a day. Or you can just wipe it off.
Green Slime/Film. This can also be solved with Plecos.
Hair/Thread Algae. This kind of algae grows in strings. The don't connect to anything.
Brush/Beard Algae. This is dull green or bluish-gray algae, but are a form of red algae. This kind of algae is hard to control. The regularly grow in clumps on leaves, driftwood, and other decorations.
Bluish-Green Sheets. This kind grows on top of gravel but isn't algae. It is colonies of cyanobacteria. This is very hard to control because it is toxic and few animals eat it.
Black-Spot Algae. It is actually a very dark red algae. It can grow into larger colonies.
Algae needs nutrients to live. It gets it nutrients from light and fish waste. This is why you shouldn't have your light on all day. Also, if you have a lot of fish, this could be a problem too. If you don't make water changes regularly, this is a problem. And if you over feed your fish, then there is definitely a problem to come.
Plants do good against algae. But plants are hard to keep up with as a starting fish keeper. Another way to control algae is get some algae eaters. Plecos do real good.
Sometimes you will have to take control. If there is algae on the glass, buy an algae scrubber to clean the glass. There are also brushes for filter tubes and for filters. You could bleach the decorations by taking them out and letting them soak in bleach. Then boil them in water for about 5 minutes. Then you can put them back in the water once they cool down.
Algaecides sometimes needs to be used. But these should be your last resort. Read direction, and use as directed.