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Discussion Starter · #1 ·
Many people in FW or SW, when they use an algae scrubber (upflow, waterfall, or horizontal river designs) for the first time, get worried when more (not less) algae starts to grow on their rocks. It seems really strange, especially when nitrate and phosphate have gone lower than before. What is happening is that phosphate is coming out of the rocks. Remember, phosphate is invisible, so you can only see the effects of it, and it always "flows" from higher concentrations to lower concentrations (just like heat does).

Example: If your room is warm, and you put a cold object on the floor, heat from the air in the room will "flow" into the object until the object and the air are the same temperature. Example 2: If you put a hot object on the floor, heat will "flow" out of the object and go into the air in the room, again, until the air and the object are the same temperature. Now suppose you open your windows (in the winter). The warm air in your room will go out the windows, and it will get colder in the room. The object on the floor is now warmer than the air, so heat will flow out of the object and into the air, and then out the window.

Think of phosphate as the heat, and your rocks as the object, and your windows as the scrubber. As the scrubber pulls phosphate out of the water, the phosphate level in the water drops. Now, since the phosphate level in the water is lower than the phosphate level in the rocks, phosphate flows from the rocks into the water, and then from the water into the scrubber. This continues until the phosphate levels in the rocks and water are level again. And remember, you can't see this invisible flow.

This flow causes an interesting thing to happen. As the phosphate comes out of the rocks, it then becomes available to feed algae as soon as the phosphate reaches the surface of the rocks where there is light. So, since the surface of the rocks is rough and has light, it starts growing MORE algae there (not less) as the phosphate comes out of the rocks. This is a pretty amazing thing to see for the first time, because if you did not know what was happening you would probably think that the algae in the scrubber was leaking out and attaching to your rocks. Here are the signs of phosphate coming out of the rocks:

1. The rocks are older, and have slowly developed algae problems in the past year.

2. The scrubber is new, maybe only a few months old, and has recently started to grow well.

3. Nitrate and phosphate measurements in the water are low, usually the lowest they have been in a long time.

4. Green hair algae (not brown) on the rocks has increased in certain spots, usually on corners and protrusions at the top.

5. The glass has not needed cleaning as much.

Since filter socks, etc don't remove any nitrate and phosphate, and waterchanges and macro's in a fuge don't remove much, most people have never seen the effects of large amounts of phosphate coming out of the rocks quickly. But sure enough, it does. How long does it continue? For 2 months to a year, depending on how much phosphate is in the rocks, how strong your scrubber is, and how many other phosphate-removing filters you have (GFO, carbon dosing, etc). But one day you will see patches of white rock that were covered in green hair the day before; this is a sure sign that the algae are losing their phosphate supply from the rocks and can no longer hold on. Now it's just a matter of days before the rocks are clear.

31 Posts
Sorry for my lack of understanding, but does this apply to all rocks, igneous rocks, metamorphic, etc. I have many types of rocks and tried researching most. I have alot of slate that may breakdown overtime, I don't know for sure. I have had it for a long time. Thanks.

729 Posts
interesting theory, ...

i've made plenty of assertions in the past on other subjects, other forums, about what i thought was right and/or impossible to find out after i was wrong, ... but as i read your theory, i'm thinking, ... interesting, can't say i agree it's possible though.

unless the rock is very porous, and only then the surface area that is subjected to water, and only then if the rock has a significant phosphate levels, ... that's a lot of "if's"

just under the surface of the rock, nothing is going to diffuse into the water column, ... think of it like paint, you could paint near anything (hard to paint powder) and nothing will move past the layer of paint into the water, rock is more solid than paint

even porous rock, it's got an exponential increase in surface area, but still it's only going to be the surface area that has contact with the water.

if there is any amount of phosphates or phosphorous on the surface of the rocks this could easily dissolve into the water column.


and then there is the organic processes

bacterial activity on the surface can slowly work at 'eating' the surface of the rock, i have no idea how fast or slow this process is, ... i think the rock erodes due to being dissolved in water faster than the work bacteria does to it's surface

and the algae scrubber and mesotrophic and oligotrophic nutrient levels in the tank
a well maintained algae scrubber will at best reduce nutrients in the display tank to the lower end of mesotrophic nutrient levels, maybe a touch lower, ... but still not low enough to keep the more sensitive corals that algae could easily kill your corals in.

all algae eventually dies
and algae grows and produces new algae
when nutrients are abundant more algae grows than dies off
when nutrients are in equilibrium algae dies off as quickly as it grows more (in balance)
as nutrients are even lower algae will be dying off faster than it's growing, ... and this level is going to be oligotrophic nutrient levels, its' just not sufficient to maintain an algae scrubber

as algae dies off and breaks down organic phosphates (and other nutrients) are released into the water column. organic phosphates settle out to rest wherever they rest, water flow can try to keep these from settling in the display tank so the skimmer can pick it up, ... what isn't collected by the skimmer bacteria has first dibs at ...breakfast, lunch, dinner midnight snacks ... depends on the time of day... the bacteria processes the organic phosphates, and releases inorganic phosphates that are water soluble to be once again picked up by your algae scrubber. ... or if you have a GFO reactor to reduce inorganic phosphates as they bond with the iron.

or you could use calcium carbonate that is common in so many substrates that will bond with the inorganic phosphates ... removing them from the water column, ... this is a so-so fix, as the aragonite will slowly disolve (releasing the phosphates into the water column again, - likely to bond with other aragonite surfaces) or bacterial activity will process these phosphates and again release inorganic phosphates again, ... (same story over again) although once the aragonite is saturated it has no where to go but back into the water column. also why many people with reef tanks resort to bare bottom tanks (no substrate at all) and rely extensively on water flow to ensure that nothing settles in the display tank to be picked up and removed in other ares (algae scrubber, GFO reactor, skimmer, etc.)

if your really intent on removing phosphates and making that the limiting nutrient for your tank fine, algae will be affected, ...

the initial algae bloom is due more to bacteria suddenly having access to organic phosphates more than what's being leached out of rocks, ... that don't leach any nutrients more than what the water has access to on it's surface, as the bacteria runs out of organic phosphates inorganic phosphates are released in abundance into the water column, ... and algae blooms are seen


the mechanical aspects that go on in our tank are rather minimal in the overall processes of what is going on
the organic processes, bacterial, algae, fish, snails, other, are the majority of everything happening in our tank, the little things we can't see but that are doing most of the work

things being dissolved, - mechanical
phosphates diffusing out of the rocks in our tanks to bring balance and equilibrium - an impossibility
bacteria releasing phosphates into the water column, ... will be releasing phosphates that algae and plants have easy access to grow more with.


tests for phosphates, ... if your after a specific level (above zero) sure they can help you out here.
last i heard they're good down to 10, with a +/-15, ... so you could be reading 0 and have an actual of 15, ... even though they say they're good to read down to 10, ... ... and people will be reading these results as zero because that's as low as the test goes.

once levels reach minimum sensitivity for your tester, your tester is useless

and oligotrophic levels, ... your tests are useless, your better off paying attention to any algae blooms in the tank and saving yourself $$$
sure test for mesotrophic levels (and higher - eutrophic, ... if you've got hypertrophy levels why bother testing, you've got algae blooms or you've got a really healthy planted tank)

if you've got coral, do what you can
if you've got sensitive coral, ... well the assumption is either your a newbie and your coral is going to die as a horrible heartbreaking experience, or your staying as far away from algae scrubbers as you can.


sources - a bunch of collage students that have access to many research papers on the subject who whent into their own extensive research to identify what goes on with algae scrubbers and why there might be an algae bloom noticed in the algae scrubber after an extensive cleaning in the tank.

you've cleaned out the food for the bacteria, the bacteria starts to drop dead all over, as the bacteria dies it releases nutrients and these nutrients are considered food for the surviving bacteria in the tank, ... there is a sudden surplus of nutrients available for algae to grow with, ... then levels once more balance and your back to algae maintaining an equilibrium , ... dying as fast as it's growing ... well growing faster than it's dying as our tank critters are being fed, and they're pooping, and this is processed by bacteria to allow for inorganic phosphates that are available to algae.


now if i'm wrong, and rocks are porous enough to allow water to diffuse into the rock, to allow nutrients to osmosis out of the rock, (till equilibrium is reached) then i apologize profusely, ... but the only things that are ever porous were dried, ... allowing gasses to escape that allowed them to dry into a solid or gasses that had to be released to allow it to cure. other then that things are solid and nothing is getting through the surface as there are no holes at any level into the deeper areas

even porous lava rock, it has a much higher surface area due to the visible bubbly nature, ... but it's still only the surface that has any connection with the water to allow anything to be dissolved
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