Separately, let's discuss such parameters of a target (not concentrated) solution as pH and EC (electrical conductivity). The first of them, pH is rather straightforward: each plant has its own preferences, and usually they are in the pH 5.5-6.5 range. Although, of course, there are exceptions.
This optimal values are in severe contradiction to, for example, the pH of tap water in most places. For example, water in my home city is 7.6. What should we do?
First, we can use filters. But even if you have your water filtered, it is not going to magically have the desired pH after you add all the necessary components.
So we go to the car dealer and buy sulfuric acid. Also we go to Alibaba and buy a pH meter:
Make sure you have figured out how to take care for this thing, rinse it and calibrate.
In addition, I'd like to note that sulfuric acid from an autoshop contains lead and some other heavy metals, although the amount, given the expected dilution (more than 10,000 times) will be lower then the sanitary standard (for a particularly country and particularly acid, find the concentration of heavy metal, the seller have to report it somewhere).
If you prefer to play it not just safe, but ABSOLUTELY safe, buy pure sulfuric acid, in which lead content is about a hundred times less. Again, availability and price of such acid depends on your country.
Note that clean sulfuric acid usually is sold as a concentrated solution, while in a car store it is usually 30%. There are certain rules about handling concentrated acids, please read them.
The second important device is the EC meter. It looks like a pH meter, except instead of glass sensor, two electrodes stick out of it. And it measures the conductivity of the solution.
On the one hand, the more fertilizer, the higher is the conductivity, so everything looks simple. On another, if you pour just salt (sodium chloride) into the solution, the conductivity will also increase - the EC sensor measures concentration of ANY salt; for example, tap water also conducts current, and conducts it well!
Therefore, it is possible that your plant will extract from the solution, for example, all nitrogen, but will leave behind all potassium. And the EU will not change much (you will add fertilizer, to compensate the vaporization, the plant will take nitrogen and water, leaving potassium ... EC can even grow!) As a result, at high EC, the plant will starve.
Therefore, it is recommended to completely change the solution every couple of weeks. "Spent" solution can be used to water plants that are not that important, for example those growing in the soil.