Hello,
You should stop trying to manage pH. That is one of the absolute worst things you can do in a planted tank and in any other tank for that matter. You also need to stop thinking rigidly about KH and GH solely in terms of their numerical values.
I think that's the biggest problem in this hobby. People become hypnotized into chasing numbers without understanding the meaning of those numbers. Any natural pH/GH/KH number can work depending on the situation. What you must learn to do is to understand what each of these numbers mean so that you can understand what the ramifications are for the health of your plants and fish.
Lets start with GH because it's the easiest to understand:
The term General Hardness for our purposes is an expression usually associated with the amount of Magnesium and Calcium dissolved in the water. Most South American, Central American and the Congo River basin tributaries are very low in Ca and Mg, and the fish have evolved to live and breed in these waters. On the other hand, the waters of the African Rift Valley are extremely high in Ca and Mg so their bodies are adapted to live in these types of waters. So if you measure the GH in a tank with Malawi chiclids the readings will typically be high because the hobbyists will put salts containing Mg, Ca and other types in the water in order to be consistent with the lake water.
For tanks containing "New World" and Congo River Basin tropicals, such as tetras, angels, discus and so forth, the situation is just the opposite, and since the waters are almost devoid of Ca, Mg and other metals, fish from these areas do better in tanks filled with water having been filtered with a Reverse Osmosis filter. We have seen however that these soft water fish can do fine in waters that have high dissolved mineral content, but for many of the more exotic species, breeding and best health is with water low in dissolved salts.
For plants there is much more flexibility in terms of dissolved salts and mineral content. Magnesium and Calcium are important nutrients and the plants seek out sources of these elements in their natural waters which are low in Ca/Mg content. When you measure GH though, this numerical value tells you nothing about the relative amounts of Ca and Mg. Normally, the water coming from our tap is high in Ca because it comes from ground waters which are in contact with limestone or other Calcium containing minerals. But if you are using RO water, or water that is naturally low in GH then that water will have a zero or nearly zero content of Ca and Mg so you'd want to add enough of both in order to ensure that the plants have access to sufficient levels to both. As it turns out, although both Ca and Mg are required nutrients, they are not needed in large amounts. They are very similar to our need for vitamins. The amount of vitamins you need are very small in comparison to the amount of starch or proteins that you need. And so it is with Ca and Mg for plants. Since they only need a small amount, and since soft water fish do better with low amounts, the general rule of thumb is to target a low GH value that ensures sufficient amount of Ca/Mg. So the numerical value of 4-6 range is often quoted, again, as a general rule of thumb, but you stlll need to ensure that there is both Ca and Mg in the water because it's entirely possible to have a GH reading of, say, 4, and to have all of it due to Ca with zero Mg, or, the reading could mean that 100% of it is Mg with zero Ca. If it's the first case then the plants would suffer a Magnesium deficiency and if it's the second case they would suffer a Calcium deficiency. If the reading was due to a 50/50 (or somewhere thereabouts) split, then that would be fine, but since we don't know the split, the safe thing to do is to add a product that we know contains both Ca and Mg. Equilibrium is such a product, but there are other cheaper products that will do exactly the same thing from a GH perspective.
On the other side of the ledger, we have seen that similar to the soft water fish, plants also have a high tolerance of dissolved salts and minerals so if your tap water is very high in GH, i.e high in Ca and Mg, then this is not a problem. There are only perhaps a half a dozen plant species that dislike high Ca/Mg content. The vast majority do not care.
KH is a indicates a different chemical content. It is normally referred to as Carbonate Hardness. There are two basic species in the expression "Carbonates". The most abundant is Bicarbonate (HCO3) and Carbonate (CO3). These two compounds have the effect of combining with acids in the water and sequestering them. This is why, when you have an upset stomach after a large meal (which causes a high acid production in the gut) you can take bicarbonate product like Tums or Alka-Seltzer. The tablet dissolves in the stomach and releases the Bicarbonates which then combines with the acid. Bicarbonate is a very effective compound in controlling the level of acid in a solution and it is this action that the term "buffer" is related to. If there is a large amount of Bicarbonate/Carbonate in the water then any acid that is produced get quickly sequestered. When we talk about "acid" what we really are talking about is the Hydrogen ion H+ which has a positive electrical charge. Bicarbonate/Carbonate both have negative charges and so they attract the H+. That is one way in which an acid is neutralized. The H+ is still there but it is much less reactive since it is in a way, tied up by the HCO3/CO3. Our bodies and fishes bodies produce bicarbonates in order to control the level of acid (H+) in the bloodstream.
From a plant standpoint, some plants use Bicarbonate to help them produce CO2, which is super important, so again, there is a general rule of thumb to try and have a sufficient amount of KH, but if you are injecting CO2 then this is much less important. In a non CO2 enriched tank it is often suggested to regularly add a bicarbonate source to help those plants produce CO2. From a fish standpoint the situation is similar to GH, in that fish from soft waters typically don't see very much bicarbonate levels and those fish from Rift Valley are immersed in high levels. The general rule of thumb is to have a high enough amount to satisfy some plants but not too high for the fish. However, we see time and time again that fish are very adaptable to a wide range of bicarbonate/carbonate levels and it is not too much of an issue unless you want to breed them.
Then, the parameter which most people worry about and which happens to be the parameter that is the least significant is pH, which is a simple ration of the amount of Hydrogen protons (H+) to the amount of Hydroxyl ions (OH-). If you connect H+ to OH- you will get HOH, which of course is water H2O. Water is nothing more than a combination of H+ and OH- which are constantly combining and separating. If you count up all the separated H+ in pure water and put that number in the pH formula you'll get the number 7. If you count up the number of free OH- and use the same equation you'll also get 7. That's because, in pure water, the number of free H+ at any one time has to be exactly the same as the number of free OH-. The difference come when we add H+ or OH- to the water.
If we put vinegar in pure water, there is a rise in the amount of H+ compared to the amount of OH-. Vinegar is a weak acid. It's also known as acetic acid and it has a formula (CH3)(CO2)H.
When you put it in the water that H on the end of the formula is pulled from the vinegar so you get H+ Hydrogen ion, plus acetate (CH3)(CO2)-. But vinegar is a "weak acid", so only a very small percentage of the (CH3)(CO2)H gets separated. But in pure water even this small amount, when used in the pH equation can give you a very low pH. Weak acids are a staple of our diet and it is what makes things "tangy". Hot sauce, citrus fruits, tomatoes, vinegar - they all taste good or enhance the flavor of things because they release only a very small amount of H+.
Likewise, in the tank weak acids do not have a negative effect on the fish, especially fish that originate from waters which have weak acids, like tannins, dissolved in the water. So the pH can go very low due to these weak acids without any effect at all.
Strong acids on the other hand are highly toxic because they disassociate almost completely, releasing massive amounts of H+ into the water which then becomes highly toxic.
You can see now from the above that when water has some KH, meaning that there is a population of bicarbonate/carbonate, and when acids (H+) are released into that water, then the two tend to cancel each other out, because they attract each other and sequester each other from the rest of the water. Water with a low KH has a low population of bicarbonates (or a low population of other chemicals that behave similarly to bicarbonates) so it does not neutralize as many H+ and so the measured pH will be lower than if we put exactly the same amount of acid in water which has high KH.
That is why it is completely insane trying to manipulate KH/pH. They are directly related and so whatever you do to one will affect the other. Also their values do not really matter. Fish and plants do not really care what the pH or KH of the water is and unless they are supposed to be breeding.
So when you see some substrate vendor rave about what effect their product has on KH it's better if you ignore it because firstly it doesn't really matter, and secondly the effects will not last forever. If a product claims that it lowers KH all that means is that it attracts and removes bicarbonate (or other chemicals which affect the alkalinity) from the water, and after a while it will become saturated with bicarbonate (or the similar chemical) and will no longer attract anymore unless there is some other factor which removes the bicarbonate or other chemical from the surface of the grain.
The best thing to keep yourself sane therefore is to simply get the substrate that you like the look and feel of, or the substrate that will feed your plants, or the substrate which is cheaper (depending on your priority) and to completely ignore the claims about KH. Similarly you should think more about "do my plants have enough Ca and Mg?" more so than blindly trying to adhere to some predetermined GH value. Lastly you should absolutely ignore the pH and absolutely NOT try to manage pH because that will cause 10 times more problems that it will ever solve.
Cheers,