What causes an algae bloom is a frequently asked question by aquarium owners, and the subject of much discussion and debate among hobbyists. Most of the answers, though, boil down to four points: light, food (nutrients), temperature, and filtration.
How much light is being fed to the algae?
When I come across an aquarium with an algae bloom, the first thing that I look at is the lighting. Light feeds the algae through photosynthesis, both the good and bad algae. So I look to see what changes have occurred.
Often, with the change of the seasons, more sunlight is hitting the aquarium from an exterior than had been before. We tend not to notice because the change is so gradual.
While light may not be “suspect number one” for what causes an algae bloom, it is the factor that as a maintenance technician, I have the most control over. So I will almost always bring the amount of light going into the tank down to its minimum levels, both in intensity and in time. The idea is to either slow down the growth to a more manageable rate, or to starve it out, entirely.
How much food is being fed to the algae?
Light is the factor I have the most control over, and food is the factor of what causes an algae bloom which I have the least control over.
Every bit of food which goes into an aquarium can feed algae and is part of what causes an algae bloom. If it isn’t directly, as uneaten food, then it is indirectly as fish waste. I strive to help strike the right balance between too little and too much. If all the food that gets put into an aquarium is eaten within a minute, or so, then whoever is doing the feeding is doing everything they can do to make sure the fish get what they need and that the algae don’t get what they need.
An additional factor is how much “fish” is in the tank, both in number and size. Larger fish eat more and produce more waste than smaller fish. The more fish an aquarium has, the more the filter is being asked to process. This applies to both number and size.
Some fish produce enough waste to account for multiple other fish, and the type of waste can have its own effect. A few chromis or other small fish won’t produce as much waste as one medium to large eel.
The amount of waste which is being generated by each fish adds up. The term used is “bio-load,” which refers to the amount of life in an aquarium, which is both creating waste and in some cases, living because of the waste.
If an aquarium has several large fish and they are getting overfed, it is an invitation for algae to take over the tank.
How much heat is feeding the algae?
Before each summer, I like to add a strong cleanup crew to every aquarium so that when the temperature shifts, the resulting algae gets eaten as opposed to getting a foothold. The temperature shift that occurs when the summer months arrive, no matter how slight, can be just enough above the established ecosystem that the aquarium doesn’t “know” what to do with this extra heat energy in the tank. So it becomes part of what causes an algae bloom and grows nuisance algae more rapidly.
This can even happen in air conditioned rooms. In my experience, I have seen air conditioners not keep up with the rise in temperature. Even though the thermostat says 78°, the room is at 80°. In an aquarium, stability for the ecosystem is a main goal, and a slight temperature change can throw it off. It’s not usually the end of the world, but it is enough for algae to grow more quickly.
Is the filtration keeping the algae fed?
The main job of an aquarium is to remove waste from the water, and it does this through its filtration system. If an aquarium’s filter is not removing waste from the water, then it is contributing food for algae to consume, and becoming another part of what causes an algae bloom.
The “hang on the back” filters are the main offenders. Over time, the filter pads can get clogged if they do not get changed often enough, and detritus waste can build up and accumulate inside the filters, themselves.
Some of these filters are being asked to do more than they can do. When someone buys a filter at a store, the box often has a size of aquarium printed on it that the filter is supposed to be able to handle effectively. The size is normally valid for an aquarium with only a few fish in it. But if there is enough “fish” to create a higher bio-load than then filter can handle, then the rest of it goes unprocessed.
Sump filters are just as guilty. The buildup I have seen in some of these filters could feed its own algae bloom for any size aquarium.