I've learned a lot over the past few years about algae, both by dealing with
aquariums and also by owning a large pond (6 acres). I thought I'd pass this
information along.

Algae is like any other living thing out there. It needs food and nutrients.
Food comes in the form of ammonia, nitrites, or nitrates. If you didn't
know, anything you put into the pond that is organic (food, topsoil, dying
plant leaves, poop from fish, etc) will decompose and emit ammonia. Bacteria
will convert this ammonia which is toxic to fish into nitrite (which is
still toxic) and other bacteria will convert this into nitrate (which is not
toxic to fish). All three can be used by plants and they are their food
source. And incase you didn't know algae is a plant.

Algae is just like anything else living, it also needs trace minerals. Could
you survive without vitamins and minerals? Nope. Trace minerals to plants
are things like iron (real important), boron, calcium, magnesium, etc. These
elements are in the water you get out of your tap or out of the ground (if
you have a well) or in the miracle grow stick you add. Topsoil also contains
trace minerals. When you change any of your pond water this is why you may
see a renewed growth spurt in your algae. This is because there may not have
been any iron left in the water and this was limiting it's growth. Then you
change the water and add new water which contains iron and algae now has
iron to grow. Different people have different concentrations of minerals in
their water. This is why one pond owner might never have an algae problem
while another just can't seem to get rid of it. Most resort to changing
their water in an effort to reduce the nitrate level but instead it just
keeps the mineral level replenished.

One thing that pond owners aren't accustomed to is the idea of "water
column." Any nutrients and minerals that enter the water column will become
food for algae. It's important to keep nutrients and minerals out of the
water column to stop algae. It's possible to have a layer of miracle grow
and fish food as your substrate but have no algae. If no water is flowing to
stir the miracle grow into the water column algae can't get it. Again you
may notice that when you clean your pond which has a gravel substrate you
get an algae bloom. This is because all the nutrients and minerals that were
locked up in the gravel are now all stirred up and in your water column. Now
that you understand about the water column you can understand that if you
have a potted plant full of topsoil (rich in organics and minerals) and 15
miracle grow sticks in it and a water current is passing over it you can see
that you are loading your water with both minerals and nutrients.

Some people utilize veggie filters to help keep their nutrients and minerals
down low. A veggie filter is a box filled with other plants (or algae) that
are busy sucking up the nutrients and minerals from the water column. Pond
water is pumped into this box. The water is then passed back into the pond
depleted of minerals and nutrients. A successful veggie filter has no dirt
and only submerged plants. If you have dirt in your veggie filter the plant
will remove nutrients from the dirt not the water column AND the dirt will
add more nutrients and minerals back into the water column as water flows
over it. Plants do not need dirt to grow. They merely use it as a source of
nutrients and minerals (terrestrial plants also use it to keep their roots
wet and to anchored themselves down). It is their food source. In a veggie
filter you want the plant to use the water column as a food source.
Submerged stem plants are built to remove nutrients from the water column
via their leaves. Some grow roots but this is only to anchor them down. If
you have emerged bog plants in your veggie filter you'd be better off
replacing them with stemmed submerged plants because the water column
nutrients must pass into the substrate to reach the roots of your bog plants
to be removed. With submerged stem plants water must merely pass by the
plant's leaves to be absorbed.

Plants also need a carbon source. They can use sodium carbonate that is
located in the water by splitting the atom. This is why your water hardness
(KH) drops over time (and your PH drops) if you don't change the water.
Plants prefer CO2 as a source of carbon. If you didn't know, CO2 is the fizz
in soda pop. A common trick that pond owners do is use a fountain or
waterfall to "shake" the water up and bubble out all of the CO2 from the
water to limit algae growth. You'll notice that around fountains there
usually isn't any algae growing. (fountains and waterfalls also add oxygen
to the water). If you have completely submerged plants they will get their
CO2 from the water. If you have low hardness in the water and are using a
fountain or waterfall they'll probably die or at the least won't do as well
as they could because there will not be any carbon source available. Plants
that are emerged get their CO2 from the air.

Lots of people get pea soup water. The best way to control it is to let it
go and not add any new water which will readd minerals. You must however act
to limit the source of nitrate. After time the mineral and nitrate source
will be depleted and the algae will die.

It's funny because in the small pond world no-one wants algae and they want
sparkling clear water. In the large pond world people want pea green water
because it's required for the chain of life. In the large pond world people
actually dump bags of fertilizer into the water. Your fish will grow much
faster if your water is green because more food will be available to them.
In a large pond algae in the water feeds bugs and daphnia. These in turn
feed your smaller fish which in turn feed your bigger fish. Without algae in
the water of a large pond there would be no food source. This is why
quarries that fill with water have crystal clear water and are devoid of
life. There is no food source for algae and thus no food source for the
chain of life.

Hope this helps.

Sam