This may be a difficult concept to understand. Naturally, you would think
that cleaner water and a spotless tank would inhibit algae growth, but this
notion is not justified.
It is important to remember that algae growth is a natural, normal process
that occurs in any aquarium. Algae grow naturally in all natural water
sources, such as ponds, lakes, rivers and oceans. If a section of the ocean
floor is swept completely clean of life by some type of force, the first
organism to start growing back is algae.
When you clean your aquarium and all of the decorations thoroughly, you
leave no algae behind. In this freshly cleaned setting, algae grow back
more rapidly than it would if you left a small amount of algae somewhere in
the tank; this is especially true in saltwater aquariums, where the bright
white coral and gravel reflect more light, causing algae to grow faster.
This occurs because if there are no algae left in the tank, there is no
competition for nutrients that the algae feed on. If you leave a few rocks
or pieces of coral in a tank with a good growth of algae on them, that
algae will consume nutrients in the water, which helps control the growth
of new algae. It is actually better for your aquarium to allow some algae
to exist somewhere in the tank, especially if your aquarium has chronic
The best way to explain this principle is to look at two types of aquariums that rarely have trouble with heavy algae growth; freshwater aquariums with live plants, and saltwater live-coral or reef aquariums. Because the plants in this tank are living, they compete with algae for light and nutrients. Algae, being a relatively primitive organism, can’t compete against the more advanced plants or photosynthetic corals in these types of aquariums.