I kind of just take each post as they come so I'm not aware of your troubles. But maybe I can offer some general pH stuff.
Stable pH is more important than any specific pH value.
Measuring pH is more tricky than most people think, and it tells less than most people think. Measuring KH tells you a lot more. It tells you the general pH range and more importantly it tells you what your pH will be tomorrow.
Your tap water rise in pH...lots of possible causes. First up is the dechlorinator. Anytime you add something to water pH is likely to be effected. Second up to bat is CO2. Tap water can be high in CO2, that acts as an acid and lowers pH. Once exposed to the air some CO2 out gases and pH rises.
GH
GH is generally not tested by people because adjusting GH and the effects are complex and generally not needed to keep fish alive.
To understand GH you need to understand KH first...
To make things a bit easier lets say KH measures the amount of baking soda in your pond. Without any KH the pH can swing, hour by hour, from say 4.5 to 12. Those fast changes in pH stress fish and can lead to dead fish.
As you add baking soda the KH level may not actually increase because the baking soda is being consumed by acid. If you keep adding baking soda the acid becomes all used up and KH will start to increase. At this point pH becomes kind of stable at lets say 7.5 pH, a great pH for Koi and Goldfish. Problem is acid is always being produced in a pond, plus rain is an acid (2.0 to 5.5). As that acid enters the pond it reacts with the baking soda using it up and KH starts to go down, but pH will stay pretty stable. This is why it's called "ph buffering". KH stops acid from lowering pH. However, as KH declines there comes a point when the "buffer" is no longer working and pH suddenly drops. We call that a pH crash, and pH will start swinging again too, and not car keys in a hat kind of way.
So, what some people do is add enough baking soda so KH is in the 100-300 ppm range. KH has to be extremely high, like 5,000-10,000 ppm, to be a problem for fish. Getting KH to say 200 ppm gives the pond keeper some breathing room. They don't have to worry about a crash in the short term. They can measure KH in a week and see where it is. If its 190 they know they maybe only have to check it once a month. If it's 100 they know they have to check it every week or more.
Hopefully you can see why knowing KH tells you so much more than pH.
However, as you add more baking soda and KH increases so does pH. What baking soda does is stop pH from falling, it doesn't stop pH from rising. The result of just getting KH up is pH will rise into the 8.5 - 10 range. That's pretty stable pH. Pond fish can deal with that pretty well. Most people who get this far call it a day here. Good enough, crack open a beer,
Goodnight Irene.
Different parts of the world have different levels of KH in their source water. People in areas with high KH source water and low fish loads never have to learn about all this. Often they'll say "oh you don't need to worry about KH and all that non-sense" thinking if they don't then certainly no one else does either. People with low KH source water like you do have to be concerned...normally starting the day after all their fish die.
Now for GH...
A good GH level, say 100-250 ppm stops the baking soda from pushing pH above about 8.5. The net result is KH stopping pH from going below 8.3 and GH stopping pH from going above 8.5. Result is a very stable pH of 8.3 to 8.4.
I don't know if the chap you spoke to knew what hie was talking about, but he may have. GH could be stopping your water from going above 8.5. However, for GH to do this there must be KH, and of course a good GH level. Good GH and zero KH results in pH swings. GH on it's own has little effect.
You may often read people write, my posts included, that adjusting KH gives a pond stable pH of 8.3 or 8.4. But that assumes a good GH level so not he whole story. But most people don't really want the whole story either.
KH is a "temporary buffer" and GH is a "permanent buffer". GH isn't consumed. So generally it needs little or no adjustment once set. KH has to be adjusted, sometimes often. Whether the adjustment is adding baking soda or new source water.
Adjusting KH and GH...
Baking soda is one way to raise KH. There are many other ways, oyster shells, egg shells, plaster, limestone, concrete, on and on. These are are commonly touted by more inexperienced keepers because they sound like more friendly things. Who doesn't like an oyster shell? However these are very poor pH buffers (some aren't even buffers at all, just sound like they should be) because it takes the acid a while to get to the shells and then to break it down. Baking soda is dissolved in the water ready to bump into any bit of acid and almost instantly cancel out the acid. So with just oyster shells you can see a crash after a thunderstorm, but pH will recover, so the fish aren't completely stressed out. For people who care about really stable pH baking soda is a reasonable choice.
Things like oyster shells can be made better pH buffers. Crushed oyster shells, called grit and sold as a supplement for chickens, is a much better buffer. More surface area = faster acid reduction. Crushed into a power is even better but that's basically the same as baking soda which is cheaper and easier to find. Placing the oyster shells in moving water can help a little too.
People really get skittish when it comes to adjusting GH which is another reason people don't bother with GH. GH measures magnesium and calcium. Epsom salt is a reasonable source of magnesium. Gardeners you know about adding Epsom salt to plants may not have a problem adding this to ponds but many people do because it doesn't sound right.
A good source of calcium is calcium chloride flake...the de-icing stuff at the hardware store used to sprinkle on the sidewalk to melt ide. Few people can bring themselves to add that to their pond, but it is safe if you read the ingredient list to confirm.
Why KH is good beyond pH buffering...
The bacteria that converts ammonia also needs O2 and carbon to complete the conversion. Low KH can lead to high ammonia and nitrite levels, deadly. Combined that with pH swings and the fish haven't got a chance.
Keepers with large fish loads have bio filters that convert a lot of ammonia, using up KH really fast. They can be adding a pound of baking soda a week to keep up.