There is only one pH, so any pH you read about is the same as any other. The problem is not that there are different kinds of pH, but instead that people really don't understand what pH is and under what conditions pH is relevant. The same can be said of the water's mineral content.
The best way to think about pH is as a ratio of positive electrical charges due to ionized Hydrogen (H+) versus the negative electrical charges due to an ion called Hydroxyl (OH-). When the two charged particles combine, we have HOH...otherwise written as H2O. If there are exactly the same amount of (H+) and (OH-) then all the positive charges are balanced by all the negative charges.
Here are some schematics:
Normally, when we describe a Hydrogen atom it exists as a single positively (+) charged proton in the middle. This (+) charge has the power to hold a single negatively charged electron (-) within a certain space surrounding the proton. As an assembly, this combination has a net neutral charge. The (+) and the (-) cancel each other but the electric fields made by each are still present.
The electron is trapped in this "orbit" because of the positive electric field of the proton and there are only two ways to escape this area. One way is to acquire more kinetic energy to enable it to fly away, and the second way is to have a stronger (+) field nearby to pull it away. All of chemistry and all of life is about the movement and energy state of that little orange ball depicted in the image above. When the electron is in the orbit the material that is Hydrogen is a completely different material than when the electron leaves that orbit. When it leaves, the Hydrogen atom becomes a simple proton with a (+) charge.
Here is a scematic of a Hydrogen atom becoming a Hydrogen ion:
This (H+) charged particle is now capable of attracting other electrons and pulling them from their current orbit back into an orbit around itself. In so doing the material of the Hydrogen changes it's properties AND the material that the electron came from also changes it's properties.
The material that the new electron came from is said to have been "Oxidized". The (H+) is said to have been "Reduced". This is what acids do. They Oxidize other materials by ripping off the electrons that those materials have.
The more (H+) that are in the water the more damage they can do because the total strength of their combined (+) charges is powerful enough to break down other substances.
On the opposite side of the fence, there is the Hydroxide ion. This is a little more complicated because it's composed of two atoms; an Oxygen atom that holds a Hydrogen atom firmly in it's grasp. There is an extra electron in this assembly. Count the number of (-) red = 10 and compare with the total number of P (+) = 9 so the net charge of this assembly is -1. The Hydroxide ion repels electrons and is attracted to positively charged ions.
If there are more of these ions in the water than (H+) ions, then the solution is said to be alkaline.
In the water, because of these balance of charges there is a continual merger of the (H+) and the (OH-) and immediately they separate. If the pH of the water is neutral (7) then there is an equal number of these ions, and they merge and disassociate at an equal rate.
Now when we start adding things to the water the balance is upset. The pH calculation is a mathematical computation of the number of (H+) in relation to the number of (OH-). Here is a simplified schematic:
The difference in acids is that weak acids such as citrus, vinegar and carbonic when we add CO2 is that they only add a small amount of (H+) to the water, so when you calculate the pH, the number can be driven very low, but the amount of extra (H+) relative to the current amount of (OH-) is really not that much. Weak acids dissolved in water can dramatically change the pH, but the electrolytic effect against other substances is not that great because there are still plenty of (OH-) in the water to pull the (H+) away.
Strong acids however are highly toxic because they add huge amounts of (H+) to the water and overwhelm the number of (OH-). Now, there is real danger to fish, plants, bacteria and everything else because the (H+) protons will rip through their materials, will pull electrons away due to the strong positive charge, and in so doing, will change the nature and properties of the living material.
So when you approach the analysis of your tanks pH, you need to understand WHY the pH is high or low, because the number is just a calculation. If the number is low due to strong acid release, this is a major problem, but if the number is low due to a weak acid addition then it's meaningless.
That's why we can add CO2 and change the acidity by a factor of 10 without any problems, because the total number of damaging H+ charges relative to the balancing OH- charges is minor. People don't realize this very important difference and they add strong acids like "pH Down" to their tank, trying to control pH to some mythical perfect value, which is actually highly toxic because it's a strong acid.
Your tank produces tiny amounts of strong acids. Nitrate is metabolized into Nitric acid, Phosphate to Phosphoric acid as well as some other organic acids. Again, these are very small amounts so they do have an effect on the pH but their total contribution of H+ is not that much because the
quantity is small.
Without a clear understanding of the fundamental principles, there is no hope of making rational decisions regarding the water's chemistry. The same is true for conductivity and nutrient levels. People have no idea what effects there are on fish, so they just guess because they "feel" that something is right or wrong. While it seems a certainty that soft water fish are happier in soft water, it is also true that they can adapt to higher conductivity and mineral levels. If that were not true then there would be no way discus would breed in hard tap water, and clearly, they have. So the fish have mechanisms to deal with all sorts of things such as CO2, conductivity, calcium and so forth. When people have a problem they immediately blame nitrates and phosphates without having any evidence. There is plenty of evidence that keeping your tank clean and free of organic pollution goes a lot further to maintaining top health than stressing over nitrates, phosphates and TDS.
Again, to approach rationally, there should not be a need to use multiple EI dosing, but doing so does no harm. If growth performance using standard EI dosages do not achieve the expected performance then this tells you that you have a flow or distribution problem and that is where the most logical corrections should be made. But if that is not possible due to other constraints then a dosing increase can be of benefit without risk.
Cheers,