Periphyton is what turns your rocks different colors. You know... the white rocks you started with in saltwater, or the grey rocks (or brown wood) you
started with in freshwater. 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, with different colors, and thicknesses. And the important part is: It is LIVING. And the thicker it
is, the more natural it is.
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". Have you ever slipped while walking on rocks in a stream? That's
probably the periphyton that made it slippery. It can be a very thin coating on the rocks, sometimes paper thin, but it covers the entire surface.
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 mostly on the illuminated 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 caves and in holes, in the dark areas? Well these periphyton don't get much light, so they are primarily filter feeders. So
they REALLY grow and position themselves to be able to intercept food particles. You'll sometimes see little tree-like arms or branches sticking
out to get the particles; these are usually "forams", otherwise known as foraminifera. They require a lot of food particles in the water. Think of
them as coral polyps without the coral. The photo with the shrimp shows a lot of forams.
Reef studies have shown 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 because the water is so shallow. So, what
you have on rocks that are "mature" or "established" is a well-developed layer of periphyton, and all the good natural things that comes from it like
nutrient absorption and food generation. Nice colors too.
This is why mandarin fish can eat directly off the rocks of an "established" tank (tons of pods grow in and consume 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 (the periphyton is a giant, adjusting filter). 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. That's why there are no pure white rocks in natural reefs. Keep in mind though I'm not referring to
nuisance algae on rocks; I'm only referring to the layer of coloring and textures that coats the rocks, and the little arm-like structures that stick
out from under rocks.
But what happens when you "scrub all 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 of the water 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. So it's best to not do
these things.
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 some photosynthetic growth and thus will be
grown over and will partially die too. And even if the light stays the same, the direction and amount of water flow (and food particles) will change;
forams and micro 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, due only to your re-arranging of the rocks.
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 by 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: 1 to 2 years. Even changing the direction of a powerhead will affect the food particle supply in the area it used to be
pointed at. This is why "mature tanks" take 1 or 2 years to develop.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. In other words, treat them just like the rocks on a reef. It's a different way of thinking,
but you should have a stronger natural filter and food producer because of it.
-Santa Monica Filtration
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