how old is your system? PH shold start going down on its own with age. what is your alkinlinity?
How many gallons? what is your water source? how much you adding every week, what plants are you growing ideal fish PH is 7.5
Time!!!! Give it time.
If you can start collecting some rain water for some of your top ups you will have less issue with bumping your pH up by adding well or city water. I get my water from a limestone aquifer when natural rain isn't topping up the system on it's own. The well water will bring my pH up. But the natural action of the bacteria working the nitrogen cycle will use up some of that hardness and bring the pH down in time if you are not having to add too much of that really hard top up water.
A starting pH between 7.6 and 7.8 is fine. Just cycle the system up and keep an eye on the pH over time. I noticed that as long as you don't have a strong buffer mixed into your media, there is a good chance at about the time the nitrite spike from fishless cycling drops, so could the pH. Don't let the pH drop too far since if it gets too much below 6 you could stall or even kill off your bacteria.
I experienced some of the same problems you have described. My tap water is around 9 pH, 12 grains on the hardness scale. I was using muratic acid to regulate and pH would change slightly and then all of a sudden I would have a drastic swing in pH. I killed some fish this way even though I was treating a bucket at a time and testing everything before adding to system. I had a hard time finding the right combination. I purchased a bottle of pH down (phosphoric acid?) and tested in a five gallon bucket. I found that if I draw a 5 gallon bucket and let it sit(to room temp, 56 degrees) then add 1 Tbs. of pH down the water is 7.5 every time. I usually let this sit for another day and check again before adding to system. When I move the system outdoors again I will go through the process again to find out if/how the warmer temps affect it. The system has been humming along with no hiccups(knock on wood) for around three months now since I switched.
If your source water is above pH 8.0.... then your source water is carbonate buffered... usually from limestone aquifers...
Trying to treat a large volume of water in your fish tank with acid will just see your pH bounce back to the previous value... until the carbonate buffer has been exhausted....
Not good for your fish...
Topping up your tank with the same source water will just add more carbonate buffer to your system...
If you have fish in your system... then treat your top up water (a lesser volume)... as "K edmonds" is doing... then add to your tank...
Over time the pH... along with the nitrification trend to acidicy.... will lower the pH....
You can if you wish also additionally treat the fish tank itself... but only with minor amounts of acid... so as to not shock the fish with pH bounce... just don't be surprised if the pH returns to previous value... it will... until the buffer is exhausted...
Preferably... use Hydrochloric acid... muriatic acid... rather than phosphoric and/or other "pH down" products...
It's cheaper... and more effective...
The above will lower your carbonate hardness (kH)... not necessarily your general hardness (gH)... which is more a measure of the combined magnesium, calcium and other metalic ions present in your water... what's often referred to as "hard water"...
A general technical explaination...
There are two types of water hardness: general hardness (GH) and carbonate hardness (KH). A third term commonly used is total hardness which is a combination of GH and KH.
Since it is important to know both the GH and KH, the use of total hardness can bemisleading and should be avoided.
General hardness is primarily the measure of calcium (Ca++) and magnesium (Mg++) ions in the water. Other ions can contribute to GH but their effects are usually insignificant and the other ions are difficult to measure. GH will not directly affect pH although "hard" water is generally alkaline due to some interaction of GH and KH.
GH is commonly expressed in parts per million (ppm) of calcium carbonate (CaCO3), degrees hardness (dH) or, more properly, the molar concentration of CaCO3. One German degree hardness (dH) is 10 mg of calcium oxide (CaO) per liter. In the U.S., hardness is usually measured in ppm of CaCO3. A German dH is 17.8 ppm CaCO3. A molar concentration of 1 milliequivalent per liter (mEq/l) = 2.8 dH = 50 ppm.
Note that most test kits give the hardness in units of CaCO3; this means the hardness is equivalent to that much CaCO3 in water but does not mean it actually came from CaCO3.
Water hardness follows these guidelines:
0 - 4 dH, 0 - 70 ppm : very soft
4 - 8 dH, 70 - 140 ppm : soft
8 - 12 dH, 140 - 210 ppm : medium hard
12 - 18 dH, 210 - 320 ppm : fairly hard
18 - 30 dH, 320 - 530 ppm : hard
higher : liquid rock (Lake Malawi and Los Angeles, CA)
General hardness is the more important of the two in biological processes. When a fish or plant is said to prefer "hard" or "soft" water, this is referring to GH. Incorrect GH will affect the transfer of nutrients (trace elements) and waste products through cell membranes and in AP systems...
Carbonate hardness (KH) is the measure of bicarbonate (HCO3-) and carbonate (CO3--) ions in the water. In freshwater aquariums or aquaponic systems of neutral pH, bicarbonate ionsv often predominate.
Alkalinity is the measure of the total acid binding capacity (all the anions which can bind with free H+) but is comprised mostly of carbonate hardness in freshwater systems.
Thus, in practical freshwater usage, the terms carboante hardness, acid binding, acid buffering capacity and alkalinity are used interchangeably. In an aquarium, or aquaponis system, KH acts as a chemical buffering agent, helping to stabilize pH. KH is generaly referred to in degrees hardness and is expressed in CaCO3 equivalents just like GH.
In simple terms, pH is determined by the negative log of the concentration of free hydrogen ions (H+) in the water. If you add a strong acid such as hydrochloric, or nitric acid to water, it completely dissociates into hydrogen ions (H+) and its "conjugate base" or "salt".
The hydrogen ions freed in the reaction then increase the concentration of hydrogen ions and reduce the pH. Since nitric acid is the end product of the nitrogen cycle, this explains why aquarium pH tends to decrease and nitrates tend to increase over time.
When the aquarium has some carbonate buffering in it, the bicarbonate ions will combine with the excess hydrogen ions to form carbonic acid (H2CO3) which then slowly breaks down into CO2 and water.
Since the excess hydrogen ions are used in the reaction, the pH does not change very much. Over time, as the carbonate ions are used up, the buffering capacity will drop and larger pH changes will be noted.