Many dosing methods suggest that they have some way of solving all your algae issues. Curiously, such methods also fail to address algae growth and causes at the root.
While a method might work to solve the issue of a noxious algal bloom in your aquarium with plants, why it does it, is a matter of debate. Therein lies the crux of myths in the hobby and why I am writing this.
Control of Algae in Planted Aquaria
This is a method to address algae in the aquarium.
However, it is really a method to grow aquatic plants.
Aquatic plants require 3 main things to grow: light(they suggest enough, however on the low side), CO2, (also on the low side, but error in measurement and low light play a large role there as well as dependency on limiting PO4, which reduces CO2 demand, indirectly stabilizing CO2 availability to plants) and nutrients.
Since nutrients are LIMITING with PMDD, PO4 specifically, the effects on CO2 are also present. CO2 at 10-15ppm at low light and under strong PO4 limitation will work.
However, adding PO4 will induce a number of algal species in this system.
To test whether it is really PO4 limitation that is causing algae to be controlled, we must ensure that CO2 is also not limiting and scaled up when the PO4 concentration is increased.
Many aquarists see that adding PO4 or allowing it to become higher= algae blooms for their systems, yet cannot explain why other aquarist can have high PO4 without any trace of algae for years, even at low, medium and very high light intensities.
This reduced CO2 dependency due to strong limitations of nutrients (PO4 is one of the better nutrients to chose), is based on Liebig's Law of the minimum.
If PO4 is strongly limiting, adding more CO2 will do nothing to enhance the rates of growth. You can add all the CO2 in the world and the plants will not grow more, the PO4 is the bottle neck, not CO2.
Still, the aquarist who finds management of PO4 = less algae will remain unconvinced, yet have a conflict with other aquarist that have high PO4 and no algae. However, simply because they chose or cannot maintain good higher CO2 along with higher PO4, does not imply they are correct.
Some people simply do whatever works for them and are resistant to change or learning a new method that addresses balances of light, CO2 and nutrients and scaling up or down depending on light intensity etc.
There is nothing wrong with that.
However, to suggest to other hobbyists that it is the cure and the root cause for controlling algae is......well.............very poor logic and even poorer experimental test protocol.
I suggest hobbyists keep good plant growth and algae in 2 separate groups.
PMDD address plant growth perhaps more than algae control.
PPS does something very similar to PMDD, as does Redfield ratio, MCI, and few other methods aimed at controlling algae through nutrients, mostly all of them have one central component: limit PO4.
What does PO4 limitation do?
Is it really limiting algae?
No, not really.
That metric is extremely low, beyond the limits of test kits and methods used in the hobby. Less than 10 parts per Billion.
So what is occurring?
Well, using Liebig's law, we see that PO4 is more strongly limiting than CO2.
Thus the plants are better able to handle and withstand PO4 limitation, than CO2 limitation. This is a plant issue, not an algae issue.
We can go back and instead of limiting plants and forcing them to reduce their CO2 demands via PO4 limitation, we can add CO2 at a higher rate and also add more PO4 at a much higher rate. So CO2 and P4 are both non limiting.
If you only test one side of the coin, you leave yourself open to false conclusions, even if they might work to cure the algae, they do not explain why the algae is gone nor are able to explain why other observations show that high PO4 also does not induce algae.
So what does induce algae growth?
This is a different question that how to get rid of algae.
Very different and specific.
The answer/s is/are not as nice. They are specific to each and every species of algae. They involve space and time (Ecology), light, CO2........and nutrients.........as well as the status of the plants.
How might we test for algae inducement independent of good plant health?
1. You must have a reference control. This means a nice well run tank with plants, no algae and non limiting CO2 AND NUTRIENTS. Light can be easily manipulated for PAR and is the growth rate determination when CO2/nutrients are non limiting(up to a point where metabolic growth rates can longer be increased, the maximal yield).
2. Test light, go from the lowest possible to highest possible PAR. Vary and adjust the duration. Can light manipulation control algae the same way that PO4 limitation can? Certainly and better even. By reducing light intensity, this also reduces CO2 demand(there will be difference depending on how strong of a reduction light or PO4 might be for each case). Still, most aquarist have long noted reducing light intensity "cures algae". Why this is still is based around "Light= drives CO2 demand= drives nutrient demand".
If you limit nutrients strongly, then adding all the CO2 and light will not change the rates of growth, likewise, if you limit light, then attaining ample CO2 and nutrient is much easier as well, algae also grow slower, since their only limitation is really light intensity.
3. Test CO2. This is more difficult. CO2 measure is the hardest parameter to come to grips with. By adding good ample known PAR intensity for a plant, and non limiting nutrients, now we can test CO2 effectively. We may also add sediment types to this, both plain sand and enriched soils, to see if sediments play a role or not and to add as a back up for nutrient limitations/preferences within the root zones as well as the water column.
Since light and nutrients are independent, CO2 is the only dependent variable.
By adjusting CO2, we can induce and cause germination of dozens of species of algae.
Only when there is dependence on other variables with PO4 limitation control methods, do we see algae issues(Say adding PO4 to 2-3 ppm).
This suggest it is CO2, not PO4 that is controlling germination for many algae species. Perhaps.........but it might be due to plants and Carbon uptake that signals algae to start to bloom, not CO2 directly either.
Still, we know PO4 alone cannot account for algae blooms for all systems and we know we can consistently incduce algae independent of nutrient and light with CO2. We cannot consistently induce algae with varying just the PO4 when other nutrients and CO2, light are independent.
The only conclusion is that limiting PO4 is not limit algae, rather some other cause must be present. This does NOT............discount that in some planted systems, that limiting PO4 does show a reduction in algae. but that the cause is indirect and related to CO2, not PO4. to test this, we simply add more CO2 and raise the PO4 and we no longer have algae blooms.
This still does not imply cause that CO2 is the key.
It is only 1 step closer at getting at the root cause.
From a management perspective for control of algae, CO2 is central.
The same is true in a non CO2 aquarium, CO2 stability is key.
Plants have a rough time adapting to different CO2 levels, algae do not(they are virtually never limited by CO2). Plants need to make a lot more Rubsico to adjust to lower levels, often several times more(10-20X with CO2 enriched vs no CO2) and it takes time for the plant to make these enzymes.
Changing CO2 around day to day, hour to hour, week to week the plant struggles and spends more energy adapting to CO2, than with growth, acquiring nutrients, catching light etc. In non CO2 systems, changing water frequently causes a spike in high CO2 week to week etc, done only once every few months is better.
For CO2 enriched tanks(non limiting CO2 systems), changing water has no such impact. The plants do not need to adapt to CO2 since they have ample CO2 and supply for all conditions.
We may also limit CO2 slightly and PO4 and have somewhat stable systems also, but these are harder to duplicate. Still, plants will adapt moderate levels of CO2 as long as they are stable enough for maintaining the same Rubsico concentration in the tissues.
So often,m not doing water changes (or as much) in these reduced CO2 systems(like PMDD suggested 10-15ppm) will work well and appear to reduce algae. Still, these systems are still limited by CO2, the plants have adapted, just like the non CO2 systems. As long as the CO2 is stable and does not change/vary much week to week, the plants should still grow well and reduced, but algae free.
Both of the above curves show that at lower levels, the yields are reduced in the limiting ranges. It is not black and white, they have varying degrees of limitation.
Mild limitation has less effect on overall growth, while large effects on CO2.
Very strong limitation has a pronounced effect on growth and deficiencies.
The adequate and C/B zones give the aquarist the most leeway for dosing, both CO2 and nutrients, as well as light.
It is only when the CO2(or nutrients, or light) are in the D zones that we can be sure that they are independent when we test the other parameters.