Phosphate and algae.

Phosphates are a bitch hey!
Actually no, out tanks need it!

Phosphorus that we test for, that we hobbyists call phosphate or orthophosphate is an essential part of all lifes cellular needs, so it is in all life's cells!
So what was the make up of once living algae, living shrimp/plankton, living fish, living inverts of any kind, they were all made up of cells!

Once dead they can go into our corals foods, our fish foods, so from there, humans, fish and any life's cells take some and the rest comes out as waste!
That's where phosphates comes from, oh and with inmost corals, we know most of their calcium skeleton is calcium carbonate, well some of their make is calcium phosphate!

Then we know that much of our cycling bacteria likes to make, and live in a nitric acid environment that they create them selves.
Then why do we at times have to cope with high cal levels in the early days, yes, that's the nitric acid breaking down the calcium based media you are using that is housing your cycling bacteria.

So amongst so many poor hobbyists thinking that the rock needs to be cooked to get rid of the leaching phosphates, no, that was part of its make up being set free by a crude organic form of nitric acid, usually anaerobic bacteria do this one.
I suppose there is tap water, bad reef salt and so on , but this is where it naturally!

From all that, we know where phosphate/phosphorus comes from, now what consumes it/takes it out of our reef aquarium water?

There are products that are either a variation of GFO, granulated ferric oxide hydroxide, or the aluminium based products that sort of do the same thing along with the removal of some of the silica content that both achieve from with in your waters or which hair algae needs to survive.

Marine algae is where most inorganic nutrients like phosphates are reduced via osmosis/absorption through algae's flesh to help make its cellular structure and more.

What algae's are in a marine/reef aquarium?
The most common in a reef aquarium are the colours that make up near all corals, which is algae!
Those algae's are symbiotic and live with in the corals cells to not just give corals their glucose hit but providing most trace elements the corals structure is built of as well, these species are sometimes spoken as zooxanthellae, which is incorrect, there are many variations of symbiotic algae living with corals cells, this one is actually a form of dinoflagellate only.
So coral algae is the most common in a reef tank, this is what you aim to please with the correct phosphate levels and that is near non!

The inorganic phosphates are easily testable and if you can, they need to be near zero on a test for a tank that you feed quite a lot.
This is so that your corals receive substantial organic phosphates keeping both lps and sps in good condition, these are still with in organic material and not inorganic as yet.
If you are feeding your tank well, the ratio will be roughly 2 thirds organic phos and 1 third inorganic, that's a good situation for corals, anemones, decorative algae's and more that you want in a reef tank.
 
Once the nitrogen cycles bacteria is done with rotting flesh of any kind, the phos from these are now inorganic and needs to be of a slight amount in your waters.
A part from symbiotic algae types, there are the browns, the reds and the greens, all these algae's are represented in your aquarium and you can not keep them out, no matter what you do in a reef tank, though reducing them is quite easy!
Brown algae's like diatoms need silica, inorganic phos and low oxygen levels to thrive, as does the darkish red bacterium called cyanobacteria that many confuse as algae, though it does not need silica.
The green algae on the glass that remains of no real concern will never go, but can be reduced with the right methods.
 
Reducing unwanted algae and setting your tank up with the right algae forms.
There are several things you need in place to easily keep symbiotic and decorative algae in good health!
Number one, an external algae area with either or both racemosa and chaetomorpha algae's should be in play.
A good racemosa will take out toxins, carbon dioxide, phosphates, proteins and more and the whole body of the algae is classed as one single cell which has huge implications.
Chaetomorpha takes out more so nitrate due to its intense symbiotic cyanobacteria make up, also it will take out some of the others as well, just no where near as good as a full bodied green algae like racemosa.( Taxifolia should never be used in any way, in any form of marine aquarium)
The next is to enable high oxygen levels in your waters.
This can be done a little with the old school named area being well designed of a bio filter called a dry section.
The best way is to use pure hydrogen peroxide, or if not available a greater quantity of shop bought 3 percent peroxide, "always food grade"!
Add this each in amounts of 1 mill of pure and more if the lesser percentage.
More info on peroxide
https://southeastqueenslandm.aforumfree.com/t2729-hydrogen-peroxide-h2o2-for-hair-algae-and-cyanobacteria
In this thread I type about direct syringing unwanted algae!
When you do this the algae you kill with peroxide, you will immediately see bubbles come from that spot, that is the make up of the peroxide being rendered harmless and adding heaps of rich fresh oxygen into the tanks waters and the now dead algae, slowly brocken down via the cycle releasing all its make up like amino acids, harmless carbons and organic phos to name just a few of what your algae was made of and more feeding your external algae and if you have decorative algae in your aquarium, like the pictures that follow demonstrates, they will thrive along with your sps and lps symbiotic algae species.