Lowering pH requires an understanding of the chemistry involved. What keeps pH stables at any level is mostly due to the chemistry of KH. Almost 25 years ago I got my first education into this area from the FINS site. This was a more modern version, digitally speaking, from one of the earliest sites know as thekrib.com. I consider the information below to be pretty decent but it does contain some inaccuracies. You can find what is below here along with a lot of other good info:
https://fins.actwin.com/mirror/begin-chem.html
pH
pH refers to water being either an acid, base, or neither(neutral). A pH of 7 is said to be neutral, a pH below 7 is ``acidic''and a pH above 7 is ``basic'' or ``alkaline''. Like the Richter scale used to measure earthquakes, the pH scale is logarithmic. A pH of 5.5is 10 times more acidic than water at a pH of 6.5. Thus, changing the pH by a small amount (suddenly) is more of a chemical change (and more stressful to fish!) than might first appear.
To a fishkeeper, two aspects of pH are important. First, rapid changes in pH are stressful to fish and should be avoided. Changing the pH by more than .3 units per day is known to stress fish. Thus, you want the pH of your tank to remain constant and stable over the long haul.Second, fish have adapted to thrive in a (sometimes narrow) pH range.You want to be sure that your tank's pH matches the specific requirements of the fish you are keeping.
Most fish can adjust to a pH somewhat outside of their optimal range. If your water's pH is naturally within the range of 6.5 to7.5, you will be able to keep most species of fish without any problems. If your pH lies within this range, there is probably no need to adjust it upward or downward.
Buffering Capacity (KH, Alkalinity)
Buffering capacity refers to water's
ability to keep the pH stable as acids or bases are added. pH and buffering capacity are intertwined with one another; although one might think that adding equal volumes of an acid and neutral water would result in a pH halfway in between, this rarely happens in practice. If the water has sufficient buffering capacity, the buffering capacity can absorb and neutralize the added acid without significantly changing the pH.Conceptually, a buffer acts somewhat like a large sponge. As more acid is added, the ``sponge'' absorbs the acid without changing the pH much. The ``sponge's'' capacity is limited however; once the buffering capacity is used up, the pH changes more rapidly as acids are added.
Buffering has both positive and negative consequences. On the plus side, the nitrogen cycle produces nitric acid (nitrate). Without buffering, your tank's pH would drop over time (a bad thing). With sufficient buffering, the pH stays stable (a good thing). On the negative side, hard tap water often almost always has a large buffering capacity. If the pH of the water is too high for your fish,the buffering capacity makes it difficult to lower the pH to a more appropriate value. Naive attempts to change the pH of water usually fail because buffering effects are ignored.
In freshwater aquariums, most of water's buffering capacity is due to carbonates and bicarbonates. Thus, the terms ``carbonate hardness''(KH), ``alkalinity'' and ``buffering capacity'' are used interchangeably.Although technically not the same things, they are equivalent in practice in the context of fishkeeping. Note: the term ``alkalinity''should not be confused with the term ``alkaline''. Alkalinity refers to buffering, while alkaline refers to a solution that is a base (i.e., pH> 7).
How much buffering does your tank need? Most aquarium buffering capacity test kits actually measure KH. The larger the KH, the more resistant to pH changes your water will be. A tank's KH should be high enough to prevent large pH swings in your tank over time. If your KH is below roughly 4.5 dH, you should pay special attention to your tank's pH (e.g, test weekly, until you get a feel for how stable the pH is). This is
ESPECIALLY important if you neglect to do frequent partial water changes. In particular, the nitrogen cycle creates a tendency for an established tank's pH to decrease over time. The exact amount of pH change depends on the quantity and rate of nitrates produced, as well as the KH. If your pH drops more than roughly two tenths of a point over a month, you should consider increasing the KH or performing partial water changes more frequently. KH doesn't affect fish directly, so there is no need to match fish species to a particular KH.
Note: it is not a good idea to use distilled water in your tank. By definition, distilled water has essentially no KH. That means that adding even a little bit of acid will change the pH significantly(stressing fish). Because of its instability, distilled (or any essentially pure water) is never used directly. Tap water or other salts must first be added to it in order to increase its GH and KH.
My water is the opposite of yours. My pH out of the tap is basically neutral- it ranges from about 6.8 to 7.2 over the years. It is a private well and raindfall/drought conditions are what dictate the changes. However, to realize my dream of one day keeping wild Altum angels I needed to soften my water and lower my pH. I had to bring them into a pH of close to 4.0 and almost no TDS- I got the water down to about 25 ppm. Then over 6 months I gradually acclimated them to a pH of 6.0 and TDS in the 60-65 ppm. I also kept them in tea stained water which meant i could not use the tradition colormetric based testing.
I used a combination of things to manage the water: Catappa leaves, Alder cones, Muriatic acid and pure water I made with my own RO/DI unit. The DI is deionizing: "Deionizing (DI) water is a purification process that removes dissolved mineral salts (ions) like calcium, magnesium, and iron using
ion exchange resins to produce highly pure H2O. It is used for industrial cooling, laboratory, and manufacturing applications, but is
not intended for consumption."
In order to manage all of this I use two pieces of digital equipment. One is a hand held TDS meter. The other was a 3-way monitor I used on the tank. The weekly changing water I batched in a Rubbermaid garbage can I set up next to the tank. I would transfer the probes for the monitor to the can so that I could set-up the changing water parameters at the level needed to bring the tank water back into the desired balance as it would tend gradually to increase the pH and TDS between changes. Once the new water was batched I would return the probes to the tank so I could monitor the exact effect on the tank of the new water.
The TDS meters are not expensive, but my 12st Ro/Di unit ran me about $135 and the replacement which contained and extra module for trapping sediment in the water was more like $165. The monitor cost me about $135. Both modals had a charcoal, an RO and a DI module. Both produced water which read 0 TDS on the meter. In general I mixed my RO/Di and my tap at about a 50/50 mix to produce the numbers I wanted. In your case I am thinking you should experiment with the mix.
Before you spend money of any of the above I would suggest you get a hand held TDS meter and a couple of gallons of distilled water from a local supermarket. This will let you test potential mixes before you buy any pricey equipment. You should have a pH test kit and a KH test kit as well. Here is the TDS tester I have used for many years.
M Digital TDS-3 Handheld TDS Meter $14.80 on Amazon.
The above meter also reads temperature but it does so in C not F. I am and F guy and so I have a chart on the wall which shows the conversion at half degree intervals from C to F for readings from 23C to 33 C. There are hand held spot pH meters out there but I found them to be too flakey. Once the pH probe gets wet it cannot be allowed to dry out. This means it has to be stored beteween usage in a special storage solution. With the continuous monitor, that probe is always in water. Moving it between the tank and the chaning container and back is so fast drying out is not an issue.
My 3-wqy monitior cam from a company in New Zealand which makes it for the hydroponic industry but it can work fine for tanks and other applications. It measures TDS in either one of two different ppm scales or electrical conductivity. it reads temperature in either C or F and finally it measures pH. I got my unit years ago before it was integrated with Wi-FI. I do not use a smart pohone and have no use for the feature, However, this basic model is hard to find and the wi-fi ones are much more expensive. Given the cost you should make sure you need this before paying that much. Here is a picture of the model I have but mine has no Wi-Fi:
p.s. I forgot to add this:
The continuos monitor pH probe etc. needs too be recalibrated every few months. There are 3 different solutions which can be oinvolved and on;ly two are needed depending on the parameters involved. They are a 4pH, a 7.0 pH and a 10pH. Depending on the whether you are testing acid or basoc water you use only two solutions. The 7.o is always one of the two, The secodn depedns on if you are making acid water when you will use the 4.0, if you are rising the numbers to over 7.0 then your second solution is the 10.0.
Calibration is easy and instructions are included. basically you rinse the pprobe in clean water- I used the ro/di water I batched or even my tap. The you submerge the probe in the 7.0. Then you rinse the probe and repeat with thw second solution based on if the water your are making is acid and you use the 4.0 solution. If you are making water over 7 then your second test uses to 10 pH solution.
It is rare that you need to calibrate the conductivity.TDS probe but there are solutions for this sould you need to do so.
What you should take from all of the above is one of the reasons you will so often read that is it better to keep species which can thrive in your tap rather than needing to alter the parameters to keep species that need different parameters. Water chemistry becomes a lot more coplex and much more expensive to do when a real change in parameters is needed.
