One of the areas where the fish keeper's interests and the natural order of things diverge rather sharply is in the area of algae. Green algae are a common, natural form of plant growth that is generally healthy for goldfish. But algae can become very unsightly in an aquarium or pond, leaving the goldfish hobbyist trying all sorts of ways to control or eliminate it.
Without trying to be scientific about this (god forbid), there are three types of algae you will commonly see: green water due to algae in free suspension in the water, hard bright green algae that forms on surfaces, and hair algae that forms long tough strands. These are what we will discuss here. There are also things called "algae" that aren't really: brown algae, which is formed from diatoms and a slimy blue-green algae, which is a bacterial growth.
My personal philosophy: don't get too worked up about algae. It is generally healthful for goldfish. If you have a highly ornamental aquarium that you will be very upset if algae are growing on things, you probably shouldn't put goldfish in it. With the high waste output of goldfish serving as food for the algae, algae growth is inevitable. For young goldfish green water actually seems to be particularly healthful. Traditional Japanese Ranchu breeders raise their fish in controlled levels of green water, which they feel is essential to proper wen (head growth) development. In Chinese, the character for algae is part of the term for goldfish health.
The basic tenets of algal control are to limit light and nutrients. Greg Tong provided the following advice in the article on plants and goldfish:
First of all, algae is a good thing. Algae indicates healthy conditions in a tank. However, algae becomes a bad thing when it runs wild.
If your algae is taking over, scrape off as much as you can before a water change. Use the water change to siphon out floating bits. Many folks let algae grow on all but the viewing side of their goldfish tanks.
Then, there are only three practical things left to do. I call them:
In my experience, Greg's advice above is great for keeping algae in an established tank from getting out of control. But once, for example, a tank has developed green water or hair algae, limiting light and nutrients is not all that effective. It can take a month or two to get things under control via this method.
The approach for ponds and for aquariums ends up being somewhat different. Ponds are usually left to reach a natural equilibrium. A new pond, even without fish, will go through a several week period of green water. This is natural and should decrease after the first month, so don't do anything special. For long-term control, you need vascular pond plants, water lilies to shade about half the surface, decent filtration, and a nice layer of hair algae on the pond walls. Given these elements, you should not have significant green water problems.
Two issues for placing fish in ponds with extremely green water is the fluctuation of the pH and super-saturation of oxygen. Under direct sunlight, a very green, shallow pond can get so much oxygen in the water that the fish can actually start to get embolisms in their fins ("gas bubble disease"). The algae will also draw all available CO2 out of the water, causing the pH to rise. Then at night, the algae will use the oxygen and release CO2, dropping the pH (and the dissolved oxygen level) significantly. Goldfish in ponds seem to tolerate pH fluctuations surprisingly well, but if you see bubbles in fins or fish gasping at the surface in the evenings or early morning, then the algae problem needs to be addressed. In the most extreme case, a UV sterilizer can be used to kill the algae.
For aquariums, if you want to keep the fish in a well-illuminated tank, then you have to accept some algae. The interesting thing is that hair algae, the hard green algae, and green water all seem to be semi-incompatible with each other. That is, if you have one, you may have little or none of the others. To my mind, that is the secret. If you want clear water, let algae grow on all sides of the tank except the front. If you want green water, then carefully clean all surfaces and keep plenty of illumination. However, in heavily fed goldfish tanks it is certainly possible to have hair algae and green water at the same time.
Hair algae is normally an awful problem for aquariums, since it gets so long and starts to choke filters and is very unsightly. Here, for once, the goldfish keeper has the edge. Your bored, perpetually hungry little charges will learn to eat hair algae (a week without food will help) and once they do, they will keep it trimmed like a lawn to a half-inch or less. I have actually found that if you grow it on an opaque surface (like slate or colored plastic), it can form an extremely attractive background, almost like a golf green.
If you've read this far, it is probably because you have green water and can't get rid of it. It is a very strange phenomenon; tanks sometimes get green water out of the blue. Once this happens, simply doing partial water changes and reducing the lighting often seems to have no effect at all. After several trials and experiments, I have come to the following conclusions. When you get green water, the algae on the walls of the tank will diminish. You will continue to have green water until it comes back.
How do you shift the balance back to the algae on the tank surfaces? What I have found is that one or two 100% water changes per week (and complete filter cleanings at the same time) seem to give the "good" algae a chance to form on the tank surfaces once again. You need to make sure you change the water before it gets really green (if you can't look down the length of the tank and see the fish, you have waited too long) or the surface algae will lose out again to the green water. In my experience, usually a week or two is enough to shift the balance away from the green water. For brand new tanks it may take a little longer. It seems like "new glass" needs a while to become a good surface for algae growth.
Once you have your luxurious, half-inch mat of hair algae growing on the back of your tank, all I do to care for it is to hose it down with a vigorous spray when changing water.