I don't see how the water quality could be so out of control
Well it certainly is; just ask the algae
with weekly water changes
Does almost nothing; removes 20%, for one day only.
The iron they added to the carbon does absorb some phosphate.
phosguard changed out regularly
Also removes some phosphate.
I completely remove and replace the polyfil
Does not remove anything but food particles.
feed every other day, and sparingly.
It's the cumulative feeding, not the daily feeding, that caused the problem.
been established for nearly a year.
This is about when it happens. Basically, your rocks gave you a free ride, doing all the absorbing for the first year, because your exports were weak. Now the rocks are full.
This might help:
Nutrient Export
What do all algae (and cyano too) need to survive? Nutrients. What are nutrients? Ammonia/ammonium, nitrite, nitrate, phosphate and urea are the major ones. Which ones cause most of the algae in your tank? These same ones. Why can't you just remove these nutrients and eliminate all the algae in your tank? Because these nutrients are the result of the animals you keep.
So how do your animals "make" these nutrients? Well a large part the nutrients come from pee (urea). Pee is very high in urea and ammonia, and these are a favorite food of algae and some bacteria. This is why your glass will always need cleaning; because the pee hits the glass before anything else, and algae on the glass consume the ammonia and urea immediately (using photosynthesis) and grow more. In the ocean and lakes, phytoplankton consume the ammonia and urea in open water, and seaweed consume it in shallow areas, but in a tank you don't have enough space or water volume for this, and, your other filters or animals often remove or kill the phytoplankton or seaweed anyway. So, the nutrients stay in your tank.
Then the ammonia/ammonium hits your rocks, and the periphyton on the rocks consumes more ammonia and urea. Periphyton is both algae and animals, and is the reason your rocks change color after a few weeks. Then the ammonia goes inside the rock, or hits your sand, and bacteria there convert it into nitrite and nitrate. However, the nutrients are still in your tank.
Also let's not forget phosphate, which comes from solid organic food particles. When these particles are eaten by microbes and clean up crew, the organic phosphorus in them is converted into phosphate. However, the nutrients are still in your tank.
So whenever you have algae "problems", you simply have not exported enough nutrients compared to how much you have been feeding (note: live rock can absorb phosphate for up to a year, making it seem like there was never a problem. Then, there is a problem).
So just increase your nutrient exports. You could also reduce feeding, and this has the same effect, but it's certainly not fun when you want to feed your animals
And I'm guessing that you've moved your rocks around, and scrubbed them...
What is Periphyton?
Periphyton is what turns your rocks different colors. You know... the white rocks you started with in SW, or the grey rocks (or brown wood) you started with in FW. After several months or years, the rocks become a variety of different colors and textures. Why? Because the periphyton that has grown on it is a mix of different living things, of different colors, and thicknesses. And the important part is: It is LIVING.
That's right: The colored stuff that has coated your rocks is all living organisms. Sponges, microbes, algae, cyano, biofilms, and of course coralline. After all, "peri" means "around the outside", and "phyto" means "plant". Ever slipped in a slippery puddle? That's probably periphyton that made it slippery. It's a very thin coating on the rocks, sometimes paper thin.
There is a lot of photosynthetic organisms in periphyton, and this of course means that they need light; but they need nutrients too (ammonia, nitrate, phosphate). And as you might figure, they will be on the lighted portions of the rocks. And they will grow to intercept food particles in the water, based on the water flow. Just think about how sponges orient their holes for water flow; the micro sponges in periphyton do it too but on a tiny scale.
What about under the rocks, in the dark areas? Well these periphyton don't get light, so they are primarily filter feeders. So they REALLY grow and position themselves to be able to intercept food particles. And they don't really need to fight off algae, because algae does not grow in the dark, so they have no need for anti-algae tactics like plants in the light have.
Reef studies have show that at certain depths, more of the filtering of the water comes from periphyton and benthic algae than comes from the phytoplankton which filters the deeper water. And in streams, almost all the filtering is done by periphyton. So, what you have on rocks that are "mature" or "established" is a well-developed layer of periphyton; and all the things that comes from it.
This is why mandarin fish can eat directly off the rocks of an "established" tank (tons of pods grow in the periphyton), but not on the rocks of a new tank. Or why some animals can lay their eggs on established rocks, but not new ones. Or why established tanks seem to "yo-yo" less than new ones. Even tangs can eat periphyton directly when it's thick enough. Yes periphyton can also develop on the sand, but since the sand is moved around so much, the periphyton does not get visible like it does on rocks. So thick periphyton on established rocks is your friend. And totally natural too. Keep in mind though I'm not referring to nuisance algae on rocks; I'm only referring to the very-thin layer of coloring that coats the rocks.
But what happens when you "scrape the stuff off your rocks"? Well you remove some of the periphyton, which means you remove some of your natural filter and food producer. What if you take the rocks out and scrub them? Well now you not only remove more of your natural filter and food producer, but the air is going to kill even more of the microscopic sponges in it. And what if you bleach the rocks? Well, goodbye all filtering and food producing for another year. It's an instant reduction of the natural filtering that the periphyton was providing.
However, what if you just re-arrange the rocks? Well, some of the periphyton that was in the light, now will be in the dark; so this part will die. And some of the periphyton that was in the dark will now be in the light, so it will not be able to out-compete photosynthetic growth and thus will be covered and die too. And even if the light stays the same, the direction and amount of water flow (and food particles) will change; sponges that were oriented to get food particles from one direction will now starve. So since the light and food supply is cut off, the filtering that the periphyton was providing stops almost immediately from just re-arranging.
Starvation takes a little longer. The periphyton organisms won't die immediately, since they have some energy saved up; but instead, they will wither away over several weeks. So on top of the instant reduction in filtering that you get my just moving the rocks, you get a somewhat stretched-out period of nutrients going back into the water. And after all this, it takes another long period of time for the periphyton to build up to the levels it was at before. Even changing the direction of a powerhead will affect the food particle supply in the area it used to be pointed at.
So a good idea is to try to keep everything the same. Pick your lighting, flow, layout, and try to never move or change anything. It's a different way of thinking, but you should have a stronger natural filter and food producer because of it.
