There's really nothing magical about pest algae control. These simple photosynthetic life forms operate according to simple principles… availability of space, light, nutrients, and a lack of predators and competitors. You can use the analogy of weight loss here to gain insight into how algae work in captive systems… measured inputs and outputs…

We need to balance our food intake with exercise, fit metabolism and sustained activity to maintain fitness and "proper" weight. Similarly a balance can be struck in a captive aquatic environment (fresh, marine, pond) between the input (light, chemical nutrients from foods, your water, perhaps d?or…), metabolism (temperature, species make up and dynamics of the system) and "exercise" (the activities of competition and predation between the algae and all else).

Inputs and Their Controls

Given sufficient room and nutrients algae of many kinds will arise and take possibly take over an aquatic environment. No, you don't have to add them, their spores will "get into" your system from the air, tap or other source water, decorations, the water your livestock come in with… and many types can grow FAST!

There are a few countervailing strategies you can employ to limit algae proliferation, and you should employ all of them:

Nutrient limitation: Have your water tested for its nitrate, phosphate content. These compounds are the most important rate-limiting materials for fueling algae blooms. If the water has much (anything approaching 1 ppm) you would do well to invest in a water treatment tool. For small systems (a few hundred gallons) this can consist of a simple reverse-osmosis device or in-line contactor. For larger volumes, chemical filtrants like activated carbon may be employed to remove these fertilizers.

Once the system is up and going, take care to not accidentally add to your nutrient inputting by overfeeding, or using foods that are not mostly palatable and digestible. Know that anything you put in the system may well "recycle" over and over again as algae. When, where in doubt, feed very sparingly.

There are many other sources of organic and inorganic fertilizers that can get into a captive aquatic system. Take care to rinse your arms before placing them in your system, be careful about the use of terrestrial plant fertilizers around the system, scrutinize all decoration items to be placed in the water. If you have a suspicion, do at least test the unknown material in a separate tank for a couple of weeks to assess its make-up.

Light/Lighting: Match your light intensity, duration and if artificial, quality to the type of system, livestock you're maintaining. Too much light is a principal cause of induced algae problems. Timers, shading… are useful here.

Predators

There are algae eating organisms for any/all types of aquatic systems. Ask around, consult the many sources of information in books, the Internet for ones that are suitable for yours.

Competitors

Often overlooked, almost universally under-appreciated are the myriad benefits of having other forms of life present in a captive aquatic environment that will outright compete with noisome algae for space, light and nutrients. Many people are unaware that there is substantial chemical interaction between plants as well. Grow photosynthetic life that you DO want. This will greatly limit the undesirable forms.

Conclusion:

As with our own weight control efforts, keeping an aquatic system in balance involves consideration of inputs, outputs and metabolism. Your success in tilting the balance in the favor of a clean, clear, algae-reduced aquarium or pond rests with limiting nutrients, excess light and providing sufficient algal predators and competitors.