When getting up your tank, provided there is light at all (Which most likely there is,) algae will be encountered. This page will highlight the aspects of preventing and getting rid of algae and similar scum in the aquarium. This is the most common pest problem, and well deserves its own page.
Algae eating fish
The common algae eater shown here is a good cleaner up to a point and is often bought for the community aquarium. It sucks onto walls and rocks and usually is seen scraping the algae off as food. It has a quick digestion and an insatiable hunger. However, it has many drawbacks, such as the following: Many times they make up a large portion of their diet out of detritus and food at the bottom of the tank, rather than mostly algae. Thought this can be useful, a lot of times they don't really make a difference in terms of your original algae problem! Algae eaters often develop the poor habit of harassing larger, slower fish by sucking onto their sides. On top of being downright annoying to the fish and causing disease-causing stress, after some time time the algae eater's teeth meant for algae begin to scrape off the fish's protective mucus, causing an unhealthy welt open to infection. Algae eaters also get large and aggressive when older, and pretty much defeating the point, lose their taste for algae! So for many the risks of getting an algae eater as a cleaner outweigh the benefits.
Shown here is an alternative. The common pleco does the same as the common algae eater, scraping algae off hard surfaces. Many individuals also sift through the gravel, searching for particles that could otherwise pollute the tank. They, unlike the common algae eater, however lack that unstoppable appetite and usually can be seen lazing in a large area of shelter. Plecos are habitual creatures that come back to the same spot time and time again. And, although usually bought to prevent algae, these attractive creatures make a good community fish in general. Beware of size, however! These have been known to grow more than a foot long!
But even then there are many, many, many alternatives to these. Almost every catfish will do it's part in cleaning the aquarium, and, surprisingly, almost very omnivorous fish will at some point take a liking to algae (Yes, your goldfish eats algae.) The only reason they don't actively feed on it is, unlike the catfishes, they don't have a special mechanism to scrape it off of hard surfaces. Nevertheless, when they are hungry they'll give it a nibble or two.
You'll obviously encounter other organisms that feed on algae, quite a few in fact, some of which are easy to care for and do a fine job. First off we'll explain snails, which are great algae removers but breed like crazy producing humongous amounts of snails in what seems like seconds. They can cure your unsightly algae but are pretty ugly themselves. One example of this is the trapdoor snail, one of the fastest to breed. Another example is the ramshorn snail. Its bright red and appears to glow under the light. These multiply also, but at least not as fast. For saltwater aquaria there is the cerith, which also performs the useful task of combing the bottom.
A word on CHEMICALS:
At some point, when your algae problems loom ominously over your aquarium, and you can barely see into it any more to enjoy your lovely pets, you will be tempted to use algae removing chemicals. And this very well may your best option. It will do a perfect job of ridding the tank of algae; however, it has various long and short-term repercussions. The fish will have to be removed and out into a holding tank for a time, the dead algae cleaned out, the water changed, and the water chemistry painfully returned to its original friendly, safe levels. If algae is threatening your hobby as you know it, by all means, use chemicals, but a word of warning: Use with the utmost caution.
Lastly, a word. The seriousness of the problem depends on how it's treated, and if you take care to clean your aquarium of the stuff by hand every month or so, none of these methods shall be necessary, although any coldwater tank could benefit from a pleco, I think.