Roughly 90% of the time (if not more) you would want to reduce the amount of algae growth, however i want to look at how to promote algae growth in your fish tank. Although this may seem like an odd thing to do, in some instances it may be beneficial to try and promote algae growth in the fish tank, many fish and other aquarium animals thrive from eating algae so why not cut down on the weekly food bill by letting your fish and other animals eat more naturally.
Such fish and animals that would benefit from this are Tropheus cichlids, some Malawi cichlids such as mbuna types as well as snails and any other vegetarian grazer such including many types of Pleco.
The promotion of algae growth is quite simple really, just do the opposite of what all the rule books say to reduce algae growth, such as keeping the light on for longer, add more lighting or even allow the aquarium to benefit from a little natural sunlight for some of the day.
Water movement will reduce algae growth so reducing this will help promote algae growth. Be careful not to reduce it too much though, if you are keeping fish then I’d advise against this method due to the water movement helping to oxygenate the water. If it is snails that you are keeping and are trying to increase the algae growth for them then reduce water flow as much as possible.
Similarly, try putting a little flake food in to the tank every day if it is snails you are keeping and want to grow algae as the flake will just act as food for the algae and promote it’s growth.
The key to doing this beneficially is by promoting the correct type of algae growth. If there is not enough light and you will end up with ugly brown algae. Most fish will not eat brown algae, what you want is green algae. Green algae needs sufficient lighting and lighting duration. 1-1/2 watts to 3 watts a gallon/4 liters is sufficient for green algae growth if the aquarium surface that you are trying to grow the algae on is not more than 12 inches/30 cm away from the light source, and their is sufficient “food” for the algae to live on.
Light needs to be available for at least 8 hours a day, with longer being better. Algae needs nitrates and phosphate to thrive. Nitrates come from fish waste, and phosphates come indirectly from fish food. In an established aquarium with sufficient lighting, beneficial algae growth should not be a problem.