NO3/PO4 does not induce algae in marine tanks


Prolific Poster
Apr 20, 2006
Hi all and especially hi Tom :) long time no see!

I am now very much into reef aquariums and am trying to understand what induces algae and dinoflagellates in marine/reef tanks.

I am running one Red Sea Max 130 litres Soft coral reef for 2 years now, where I do 3 (25%) water changes per, never dose any extra Ca, Mg, Alk, traces... nada! I only feed fish with Artemia/Mysis and top-up the system with Osmosis water.

All corals and fish are thriving and growing.
I get no algae problems of any kind despite the VERY High NO3 and PO4.

NO3 at (over) 100ppm (Salifert test)
PO4 at 4ppm (Salifert tets also re-tested with Elos both tests showed high PO4)

Ca at 400-410ppm
Mg at 1240 ppm
KH at 9
pH at 8.2
temp 25'C

Salt used for water change Instant Ocean (low in Ca, Mg)

I am not debating NO3/PO4 damaging SPS/LPS corals or even making them look brown due to Zooxhantelle development.

All I am saying that NO3 and PO4 do not cause algae blooms in marine/reef aquariums otherwise my tank should be a Green Soup :D

If not NO3 and PO4 then what induces algae in marine environments???

Is it low O2, low Oxidation-Reduction potential, low level of CO2, low levels of Amino-acids maybe, or even proteins (which are removed via Skimmer and wc), low levels of trace elements, etc...

I have seen many reef tanks with no NO3 nor PO4 and still they get algae (or/and dinoflagellate) problems.

I started a new 180 litres Soft coral reef and for 3 month I had NO3 0ppm and PO4 0ppm. I have problems with Green Dust Algae on the glass. The algae behaves the same as the freshwater GDA. I scrape it off and it attaches it self to glass after just couple of hours.
Just a day ago I started dosing KNO3 and KH2PO4 to the system and will try to get the N and P up a bit and see how that works.

Can anyone help in resolving this mystery please :confused:
Thanks a million! :D

Regards, Dusko

Tom Barr

Staff member
Jan 23, 2005
I have not done any test in marine fish only systems, but I'd say you are correct, but they can adjust the light etc.

So what about marine planted systems?
I found PO4 rather touchy(could go to about 0.4ppm, but not much higher, a ew others also found this, mostly brown films of diatoms would bloom), NO3, not much at all.

Coral tanks have other considerations.

Since there are many siutations and trade offs in marine systems, it's hard to say what is best. Still, many think less is better in FW and Marine fish only tanks, which dominate most aquarist.

Mushrooms are tolerant of higher nutrient water, I think they do better in organically loaded systems, vs highly skimmered SPS type of systems.

So an algae scrubber, or Refuguium is good for those.
If the mushrooms take out and carpet the tank well, then they will act like plants and dominate and define the system like algae/macros will/do.

XEnia and other fast "weedy" corals make good nutrient export.
0.0ppm is not a good goal, a stable low residual for health of the critters seesm to be.

I'd say there is a much more flexible range with NO3 and PO4.
And a the bacteria, and NO3/PO4 move and shift, the noxious algae responds to that, rather than some pre set concentration that's stable.

This may be true for CO2 and algae in FW I'd suspect. The change rather than the end points are what drives it.

Once stable, high nutrients and low nutrients/lean etc, work.
I've never claimed otherwise actually. The thing is they need to be stable and given time to adapt.

But folks do not see things that way and chase and mess with their systems too much and think it's NO3, PO4, or something else all the time.
Those that take good stable care and do not monkey around with things too much often end up rewarded for their patience.

I suggets frequent water changes in coral/SPS etc tanks, but few listen. But then I do not need all that other junk, can rule out a lot more issues etc.

You can too since you have a small tank, salt is the only real cost factor, and smaller tanks are well suited.

Tom Barr


Prolific Poster
Apr 20, 2006
Thanks for the reply Tom!

I will try and do nothing with the Green Dust Algae I got in my reef tank and see will it eat it self to death ;) There must be something other than NO3/PO4 that this Marine GDA loves (note; I have no NO3/PO4 both 0ppm). I am not going to dose anything except keeping good Ca, Alkalinity and Mg and monthly wc of 10% and see how that functions.

I will post back once I see what happens!

Thanks again!

Regards, Dusko