Conflicting Advice and listening to other folks

Tom Barr

Staff member
Jan 23, 2005
We(aquatic horticulture type folks into aquariums), like any group of folks or critters, are made up of a population.

This population has obviously various degrees of success.
And failures.

Now while most will tell you to plant heavily to stop algae from gaining a foot hold, and add herbivores, do not add too much light etc, there will always be a few folks that say they did not do that and they had a successful tank.

There will also always be a few folks that claim the opposite: they will say that they d
did these things and still got algae.

However, overall, most folks do find a relatively standard pattern:


But if you are that guy at either end, and are successful without doing these things, then telling folks that you need to add lots of plants, or herbivores might go against what you are seeing.

Perhaps you are doing something else you do not realize that is contributing to your success?

It's an interesting question, and one I asked myself a long time ago.
In my case, I had high PO4 in my tap water, but I also had high CO2.
So the end result was excellent plant growth and no algae.

I was at that tail end of that graph and was having success, while most folks where not.

But it was due to so much to high PO4 causing algae are many thought at the time, it was really poor CO2, or not enough nutrients and or too much light, many other factors came into play.

So we learned a lot from examining these cases where folks had trouble and where a relatively few had successes.

So what the folks that fail continuously?
the other end of the graph...........

Perhaps they are assuming their CO2 is good, when in reality, it is not.
Maybe they do not clean their tank as much, or prune as much, or have such poor plant health starting off, that it takes a lot more work to get things back to the normal state, maybe they mixed up K2SO4 and KNO3, there are many reasons for failure, but far fewer for success.

Such aquarist need to step back and not make assumptions they are not sure about. They know other folks are doing well using high PO4 dosing, so they should consider the likelyhood that it's a conspiracy and everyone else is lying?
That would not be likely.

So you can assume they are right and you can see that PO4 alone is not causing the algae issue you have.

So then you focus on the other things you do not know, say the other nutrients. They can be dosed/added at a known dose and frequency so these can be ruled out as well.

Another factor is time.
It takes time to fix something and for the system to adjust, likewise, it takes time for things to go wrong. Something you did 1 week ago often induces an algae later. Likewise, if you fix the tank, give it time to adjust to see if what you did worked.

Light is generally pretty stable, but more will drive things faster and can make the system less stable and easy to care for.

So you are left often with CO2 in about 90-95% of the cases for your problems(at least the sampling I've done with hobbyists and helping them fix their issues over the last decade). Not only algae, but the development and growth of plants is greatly influenced by CO2. CO2 can go from 30ppm to 3ppm in less than one hour in an aquarium and represents 40+% of the total biomass dry weight you see in plants, whereas the next largest nutrients, N, is a mere 1%. Can 30ppm of NO3 go to 3ppm in a day? And hour? Is the method for testing NO3 accurate compared to CO2?

Do people typically measure CO2 each hour over each day?

But it can and does change dramatically.

So a lot of focus should be spent there, and much less on less important issues that can be accounted for like dosing NO3.

You know if you dosed 5ppm of NO3 to a tank and you know if you did a 50% water changes etc, the main uncertainty is really CO2.

You know it can change a great deal rapidly, you know nutrients like K+, PO4, NO3 all respond much slower, you know that CO2 fertilization influences rate of growth by 10-20X faster, so it stands to reason that this parameter causes most of the grief for folks.

So be careful, you might see that one person is getting lucky or is not considering everything while another is just plain unlucky and not doing anything right(but thinks they are).

To get a better feel, ask around and see.
Depending on where folks are on this curve, they will hold very different views.

If they assume that low PO4 helps solve algae issues, see how limiting PO4 influences CO2 demand from plants, do you think if you limit PO4, that the same rates of CO2 will be now required as a an aquarium with 2ppm of PO4?

The plant is now limited by PO4, not CO2.
Adding all the CO2 in the world (or NO3, or light), will not make the plant grow faster or help with algae.

Many have troubles with CO2, then reduce another nutrient and see that the problem appears to be resolved, however, this is an error. It was not due to the PO4 limitation that resolved the algae, it was the CO2 was now non limiting.

Such errors led to many myths, misunderstanding and a great deal of "hot air" being discussed in the aquatic plant hobby.

In order to better the aquatic hobby (or any field of study, business etc) and progress, we should look at things a bit more closer and get to the root causes.

We can say that there is a relationship between PO4, CO2 and algae, we cannot say what causes algae unless we manipulate things to see. If you cannot measure or assume that it's good when in fact it is not, then pooir CO2 can lead you to an incorrect conclusion. If CO2 is non limiting in the low(0.01ppm), med(0.1ppm), and high (1.0ppm) PO4 levels and we get no algae, how can we say that PO4 causes algae?

Next, we repeat this same test but this time manipulate the CO2 at low,med, high and keep the PO4 stable and non limiting.


No algae when PO4 is manipulated
Algae blooms when CO2 is manipulated

This is a relatively straight forward easy test that aquarist can do.
Then you know a lot more about what is going on, rather than relying on observations alone with this multivariable system. Our brains cannot integrate all 30 parameters at once and figure out what is causing what by watching the tank.

We need to experiment to see and investigate what is really happening.
Not guess.

There are some problems with testing though.
Folks have few replicates(often just one tank).
Most are unwilling to test to induce algae to see.
Many do not have the ability or care to do measure and maintain the set up over time.
So ask them.
Ask if they did more than one rep, actively tried to induce algae, or toxicity in their critters using NO3 or NH4, how they measured, and how sure they are about those measurements(how did they measure CO2 etc and over what frequency etc).

Tom Barr


Guru Class Expert
Sep 17, 2007
Nice write up, Tom! Althought I have had much success in this hobby, there have been a few times where I would rather not have to deal with all the fuss (algae wise). I have speculated what I thought was the problem and that is about all I could do. Hopefully, I have not passed my speculation onto others as the truth to a matter. Without doing tests, as you say, there is no way to be sure of the cause of a problem. I think it's in the mindset of scientists like youself, me, and others to not jump to conclusions. I think this thread will help people think twice about things.


Guru Class Expert
Mar 27, 2008
Great post Tom! Hopefully this will help other hobbyists think things over before following suggestions.

Tom Barr

Staff member
Jan 23, 2005
Well, it makes a good point about if you are that guy at the 5% range that is having success without adding any KNO3, or KH2PO4.

Why would you need to add it if your plants are growing great without it?
What if you have no idea that the plants could grow even better than they are right now?

Suppose you notice that after every water change, the plants do and look much better?

You might conclude more water changes = better, not more nutrients, leading you to think that less nutrients = less algae and better plant growth right?

But.........what if you later realized that the tap water was loaded with PO4, NO3 and lots of CO2?

Then what might you think?

If you leave these details out, if you do not account for each system being different and try to standardize things, you will never get far.

We are human, I over looked the tap water myself, but another member in the club did not, he tested for PO4 in the tank and the tap, and they were correlated, same for 2 different high grade test methods. Then he went home to his virtually zero PO4 tank and dosed some KH2PO4 and watched the response.

No algae, intense pearling after 45 minutes.
0.2ppm of PO4 was removed from the system in less than 24 hours.

I had no idea I was doing anything wrong.

I was that guy that added little but water changes(I did add KNO3 at the time and more traces/CO2, but most things I did where based on plant responses, not known parameters, later I tested things critically to see what they where and what I was doing).

Still, I knew that the folks claiming PO4 = algae could not be right.
Steve who had just started observing PO4 dosing in his tank, adding small pulses of 0.2ppm at a time was far more conservative.

This did not say what causes algae, but it rules out potential causes and falsifies hypothesis. This is good. That way we do not waste our time with them and it's one less thing to fret and worry about.

Then we are one step closer to understanding what causes various species of algae to bloom.

And we are one step closer to providing excellent plant health and not getting caught by a myth that side tracks us from the real goals: nice healthy plant growth.

Tom Barr