First, if your municipal water, from the tap in the house comes out at a pH less than 7.8 STOP DRINKING IT, CALL THE POLICE, or the EPA, or the STATE HEALTH DEPARTMENT, your municipality is trying to give you LEAD POISONING. All municipalities MUST buffer municipal water supply to a pH higher than 7.8, this is to prevent lead from leaching out of the pipes, especially old municipal systems have lead supply lines. Lead was not outlawed from municipal systems until somewhere in the middle 1970's or 1980's, so if municipality was built before 1990, there is a pretty high probability that somewhere in the system there are lead supply lines. Lead also occurs as a contaminant in copper supply lines, in older copper solder formula and lead is a component of the alloy brass. Lead can leach from brass fittings. So even systems free of lead pipes can have lead issues, so buffering of water supplies is really mandatory.
Municipalities buffer the water to pH above 7.8 for your safety. The process is not free, so most municipalities use as little buffer as they can get away with. So generally the amount of buffer in your water supply is not very much.
Second.
@Bonsai Nut is absolutely right. If the color of your leaves is a nice dark green to medium green you simply do not need to do anything. Even a healthy light green is okay.
No need to risk screwing things up by trying to fix something that "ain't broken".
Fact: Plants exude (secrete) organic acids from their root tips which buffer the soil water film around their root tips into the ideal pH range for absorbing nutrients. This is an active process. Yes, the plants will "fix" pH problems all by themselves without us humans having to do anything. I know the orchid literature better than "tree" literature. An endocrinologist from University of Michigan in the 1980's travelled to Sumatra. There he found Paph primulinum growing on bare limestone rock. Using a portable, research grade micro pH probe, he measured the pH of the water film on the rock and on the root tips of the Paphiopedilum. The water film on the limestone rock was pH 8.1 to 8.3. Fairly alkaline. The pH of the water film of the root tips IN CONTACT with the bare limestone was 5.5 to 5.6. Later lab studies proved that orchid root tips actively excrete buffers to adjust the water film surrounding the root tips, even if the environment is as alkaline as being in contact with bare limestone.
The REASON plants exude these buffers is to optimize the uptake of nutrients like calcium, magnesium, manganese, iron, copper, and the rest of the list of micronutrients. Nitrogen is absorbed most easily by plants as an amino acid, here fish emulsion is a good source. Then next easiest is as ammonia. The issue with humans adding ammonia is there is a fine line between a good source of nitrogen, and toxic. High ammonia is toxic to roots. The absorption of nitrate requires metabolic energy from the plant, so is the least desirable way to get nitrogen into plants, but it does work, good enough for back yard growing.
Studies elsewhere confirmed that the majority of tree, shrubs, plants in general to some degree will actively buffer the water film around their root tips to the ideal pH range. As long as your water is not so alkaline as to overwhelm the plant's capacity to buffer the soil there is nothing to worry about. Plants that come from alkaline soils have a particularly high capacity to modify their root environment. Some of these are some pines, some junipers, ginkgo, and quite a few others. Plant that are "acidophiles", plants that must have acidic soil conditions put less metabolic energy into buffering their root tip pH. Azalea, blueberries and carnivorous plants come to mind as plants which have poor ability to buffer their environment. Each species has their own level of buffering capacity. But in general, if your leaves have a good color, pH is not a problem.
The take away is, if your plants are a healthy green, there is no need to mess around with buffering your water pH.
Some of us like to "tinker" it is a human phenomena, some can not resist trying to optimize conditions. I applaud
@cmeg1 for his detailed optimization of growing conditions for his seedlings. His results are phenomenal. For those new to bonsai, and for those whose brains hurt when taking the deep dive into water chemistry, there is such a thing as "good enough". If your leaves are a normal green, your water & fertilizer combination is "good enough". Relax and enjoy your trees.
Often, one can tweak their fertilizer regime and that will compensate for the pH issue. As the whole purpose of fiddling with water pH is to maximize nutrient uptake. Here an "acid plant food" like Mira-Acid, or the "Blueberry Special" fertilizer I sell will go a long way toward compensating for alkaline water issues. A low dose, 1/4 teaspoon per gallon (0.3 militer per liter) with every watering of an acid plant food will be enough for those whose water is less than 300 mg per liter total dissolved calcium as calcium carbonate. If you have hard water, above 300 mg per liter as calcium carbonate, you might have to use up to 1/2 teaspoon per gallon of an acid fertilizer (0.6mililiters per liter)
Another solution, if you have hard water, high calcium, you can top dress your potting media with elemental sulfur. This is a common practice for blueberries. I routinely top dress blueberries in gallon containers with sulfur at about 1 tablespoon per one gallon pot. (15 ml of powder). Elemental sulfur is available from full line garden centers that cater to the home organic vegetable growers. Sulfur comes in 2 different grades for garden use. A powder that is slightly granular for adjusting soil pH. This takes a full year to dissolve, is the one that is recommended for this use. The other is the very fine powdered sulfur, used as a slurry in water to spray as a fungicide. This one dissolves faster, will have to be applied two to 3 times a year. If you use the sulfur, you do not need the acid fertilizer.
According to USDA guidelines published for the nursery industry, "Generally, water with 600 mg/liter or less of Calcium carbonate is considered "adequate" for landscape nursery use without the need to condition the water". Which again goes to support
@Bonsai Nut 's statement, "If your leaves are a healthy green, you do not need to adjust your pH".
But then again, some of us like to tinker, whether the tinkering is needed or not. In which case read threads started by
@cmeg1
