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When we set up an aquarium (planted or not) we are striving toward a sort of stable equilibrium between plants, fish (and their associated wastes), lights, foods, fertilizers and chemicals. A mature tank tends to be more stable because inputs have been adjusted by the aquarium keeper and biological/ vegetative load have reached a balance. It can be said then, that algae is the result of an aquarium trying to reach a new equilibrium. Then as aquarists (who tend to hate algae and love pristine tanks with crystal clear water) we must adjust our inputs.
For a moment lets think of our aquarium as a simple isolated system of water, fish, and plants. Fish eat food, produce waste and excrete the resulting matter into the water. Fish wastes are rich in Ammonia, Nitrates, and Phosphates. Any food that is not consumed, decomposes and eventually breaks down into similar compounds. Plants in turn use these nutrients as inputs for their growth. A tank with vigorous plant growth, quickly depletes these nutrients from the water and we even have to supplement with additional fertilizers. However, if the presence of these nutrients exceed the ability of the plants to uptake them, then they slowly build up to higher levels. Algae comes on the scene and will thrive on the excess nutrients.
We can remedy this by manually removing excess nutrients by gravel vacuuming and water changes. As a general rule, change 25% of the water every 2-4 weeks depending on bioload. However, the best way to know when to change water is to monitor nitrate levels - the final byproduct of fish waste. Change water when nitrate gets around 10 parts per million.
In our opinion, the best way to control wastes is to add live plants which will naturally oxygenate the water and remove waste nutrients
Too much light might be a factor in algae growth. Direct sunlight will almost always result in algae blooms on glass and anything algae can stick to - so keep your tanks our of direct sunlight! In a planted tank where light levels are significantly higher (greater than 2Watts/ Gallon) we must look at other ways to limit algae growth. One important consideration is the light spectrum of your lights. As flourecent bulbs age, the light that they give off changes as phosphors are burned off and the light becomes more "available" to certain types of algae and less "available" to higher order aquatic plant. So, replace flourecent bulbs every 6-12 months depending on usage intensity.
What else are you adding to your stable equilibrium? Are you adding too much fertilizer for your particular system? Are you adding too much iron, too much trace, too much potassium, trace elements, phosphates, or nitrates? Too much of any or all of these nutrients can contribute to an algae problem. I suggest regular testing of nutrient levels and constant monitoring of your aquarium. This just means look at your tank and notice subtle changes before they become bigger problems. It's obvious that too much of every nutrient can cause algae but a shortage of even one can cause algae due to the idea of "nutrient limiting." For example, your tank is rich is every nutrient except for potassium. Maybe your plants must use potassium (K) intensively for growth. On the other hand, a particular type of algae needs very little potassium and thrives on high levels of Nitrates and Iron. This means your aquarium has become very accomodating for growing algae and has stymied the growth of your aquatic plants. What should your nutrient levels be? I cannot give a specific answer as it is different for each system and you will have to determine that by observing growth rates and changes in equilibrium. I will address some guidelines in another article.
One last note, many pH adjusting chemicals on the market use phosphate has a buffering agent. Be cautious of using such products especially if in a high light situation and you are unaware of existing phosphate levels.